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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2019	Dec. 12, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Registrant Name	89bio, Inc.
Entity Central Index Key	0001785173
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		13,788,982
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Title of 12(b) Security	Common stock, par value $0.001 per share
Trading Symbol	ETNB
Security Exchange Name	NASDAQ
Entity File Number	001-39122
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-4946844
Entity Address, Address Line One	535 Mission Street
Entity Address, Address Line Two	14th Floor
Entity Address, City or Town	San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94105
City Area Code	415
Local Phone Number	500-4614
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets (Unaudited) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 16,188,000	$ 11,234,000
Restricted cash	38,000	23,000
Prepaid and other current assets	362,000	59,000
Total current assets	16,588,000	11,316,000
Property and equipment, net	49,000	33,000
Deferred offering costs	1,628,000	0
Deferred tax assets	89,000	20,000
Total assets	18,354,000	11,369,000
Current liabilities:
Accounts payable	3,571,000	1,509,000
Accrued expenses	2,992,000	1,173,000
Convertible preferred stock liability	16,016,000	1,671,000
Total current liabilities	22,579,000	4,353,000
Commitments and contingencies (Note 5)
Convertible preferred stock and convertible preferred shares	49,746,000	23,073,000
Stockholders’ deficit:
Common stock	1,000
Ordinary shares		1,000
Additional paid-in capital	341,000	118,000
Accumulated deficit	(54,313,000)	(16,176,000)
Total stockholders’ deficit	(53,971,000)	(16,057,000)
Total liabilities, convertible preferred stock, convertible preferred shares and stockholders’ deficit	18,354,000	11,369,000
Convertible Preferred Stock
Current liabilities:
Convertible preferred stock and convertible preferred shares	49,746,000	0
Convertible Preferred Shares
Current liabilities:
Convertible preferred stock and convertible preferred shares	$ 0	$ 23,073,000

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019
Operating expenses:
Research and development	$ 6,680	$ 3,289	$ 9,989	$ 14,154
General and administrative	1,518	497	765	2,875
Total operating expenses	8,198	3,786	10,754	17,029
Loss from operations	8,198	3,786	10,754	17,029
Other (income) expenses, net	10,470	379	784	21,022
Net loss before tax	18,668	4,165	11,538	38,051
Income tax expense	57			86
Net loss and comprehensive loss	$ 18,725	$ 4,165	$ 11,538	$ 38,137
Net loss per share, basic and diluted	$ (30.63)	$ (6.81)	$ (29.79)	$ (62.39)
Weighted-average shares used to compute net loss per share, basic and diluted	611,226	611,226	387,334	611,226

Condensed Consolidated Statements of Convertible Preferred Stock, Convertible Preferred Shares and Stockholders' Deficit (Unaudited) - USD ($)	Total	Ordinary Shares	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Convertible Preferred Shares	Convertible Preferred Stock
Beginning balance at Jan. 18, 2018						$ 0	$ 0
Beginning balance, shares at Jan. 18, 2018						0	0
Beginning balance at Jan. 18, 2018	$ 0	$ 0	$ 0	$ 0	$ 0
Beginning balance, shares at Jan. 18, 2018		0	0
Share-based compensation	0	$ 0	$ 0	0	0
Net loss and comprehensive loss	0	$ 0	$ 0	0	0
Ending balance at Mar. 30, 2018						$ 0	$ 0
Ending balance, shares at Mar. 30, 2018						0	0
Ending balance at Mar. 30, 2018		0	0
Ending balance at Mar. 30, 2018	0	$ 0	$ 0	0	0
Issuance of ordinary shares	11,000	$ 1,000	0	10,000	0
Issuance of ordinary shares, shares		611,266
Issuance of convertible preferred shares, net of issuance costs						$ 14,027,000	$ 0
Issuance of convertible preferred shares, net of issuance costs, shares						14,900,000	0
Conversion of convertible note into preferred shares						$ 100,000	$ 0
Conversion of convertible note into preferred shares, shares						100,000	0
Share-based compensation	0	$ 0	0	0	0
Net loss and comprehensive loss	(7,373,000)	$ 0	$ 0	0	(7,373,000)
Ending balance at Jun. 30, 2018						$ 14,127,000	$ 0
Ending balance, shares at Jun. 30, 2018						15,000,000	0
Ending balance at Jun. 30, 2018		611,226	0
Ending balance at Jun. 30, 2018	(7,362,000)	$ 1,000	$ 0	10,000	(7,373,000)
Share-based compensation		0	0	0	0
Net loss and comprehensive loss	(4,165,000)	$ 0	$ 0	0	(4,165,000)
Ending balance at Sep. 30, 2018						$ 14,127,000	$ 0
Ending balance, shares at Sep. 30, 2018						15,000,000	0
Ending balance at Sep. 30, 2018		611,226	0
Ending balance at Sep. 30, 2018	(11,527,000)	$ 1,000	$ 0	10,000	(11,538,000)
Beginning balance at Dec. 31, 2018	23,073,000					$ 23,073,000	$ 0
Beginning balance, shares at Dec. 31, 2018						24,000,000	0
Beginning balance at Dec. 31, 2018	(16,057,000)	$ 1,000	$ 0	118,000	(16,176,000)
Beginning balance, shares at Dec. 31, 2018		611,226	0
Share-based compensation	37,000	$ 0	$ 0	37,000	0
Net loss and comprehensive loss	(4,887,000)	$ 0	$ 0	0	(4,887,000)
Ending balance at Mar. 30, 2019						$ 23,073,000	$ 0
Ending balance, shares at Mar. 30, 2019						24,000,000	0
Ending balance at Mar. 30, 2019		611,226	0
Ending balance at Mar. 30, 2019	(20,907,000)	$ 1,000	$ 0	155,000	(21,063,000)
Beginning balance at Dec. 31, 2018	23,073,000					$ 23,073,000	$ 0
Beginning balance, shares at Dec. 31, 2018						24,000,000	0
Beginning balance at Dec. 31, 2018	(16,057,000)	$ 1,000	$ 0	118,000	(16,176,000)
Beginning balance, shares at Dec. 31, 2018		611,226	0
Net loss and comprehensive loss	(38,137,000)
Ending balance at Sep. 30, 2019	49,746,000					$ 0	$ 49,746,000
Ending balance, shares at Sep. 30, 2019							44,000,000
Ending balance at Sep. 30, 2019			611,226
Ending balance at Sep. 30, 2019	(53,971,000)	$ 0	$ 1,000	341,000	(54,313,000)
Beginning balance at Mar. 30, 2019						$ 23,073,000	$ 0
Beginning balance, shares at Mar. 30, 2019						24,000,000	0
Beginning balance at Mar. 30, 2019	(20,907,000)	$ 1,000	$ 0	155,000	(21,063,000)
Beginning balance, shares at Mar. 30, 2019		611,226	0
Issuance of convertible preferred shares, net of issuance costs						$ 25,095,000	$ 0
Issuance of convertible preferred shares, net of issuance costs, shares						18,826,389	0
Share-based compensation	74,000	$ 0	$ 0	74,000	0
Net loss and comprehensive loss	(14,525,000)	$ 0	$ 0	0	(14,525,000)
Ending balance at Jun. 30, 2019						$ 48,168,000	$ 0
Ending balance, shares at Jun. 30, 2019						42,826,389	0
Ending balance at Jun. 30, 2019		611,226	0
Ending balance at Jun. 30, 2019	(35,358,000)	$ 1,000	$ 0	229,000	(35,588,000)
Issuance of convertible preferred shares, net of issuance costs						$ 1,578,000	$ 0
Issuance of convertible preferred shares, net of issuance costs, shares						1,173,611	0
Temporary Equity, Effects of the Reorganization						$ (49,746,000)	$ 49,746,000
Temporary Equity, Effects of the Reorganization, shares						(44,000,000)	44,000,000
Effects of the Reorganization	0	$ (1,000)	$ 1,000	0	0
Effects of the Reorganization, shares		(611,226)	611,226
Share-based compensation	112,000	$ 0	$ 0	112,000	0
Net loss and comprehensive loss	(18,725,000)	0	$ 0	0	(18,725,000)
Ending balance at Sep. 30, 2019	49,746,000					$ 0	$ 49,746,000
Ending balance, shares at Sep. 30, 2019							44,000,000
Ending balance at Sep. 30, 2019			611,226
Ending balance at Sep. 30, 2019	$ (53,971,000)	$ 0	$ 1,000	$ 341,000	$ (54,313,000)

Condensed Consolidated Statements of Convertible Preferred Stock, Convertible Preferred Shares and Stockholders' Deficit (Unaudited) (Parenthetical) - Convertible Preferred Shares - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Jun. 30, 2018
Temporary equity, issuance costs	$ 0	$ 0	$ 235
Temporary equity, partial settlement of convertible preferred stock liability	$ 404	$ 6,269
Temporary equity, recognition of convertible preferred stock liability			$ 638

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	8 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (11,538)	$ (38,137)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	4	12
Share-based compensation		223
Deferred tax assets		(69)
Revaluation of convertible preferred stock liability	755	21,018
Changes in operating assets and liabilities:
Prepaids and other current assets	(53)	(314)
Accounts payable	1,352	1,610
Accrued expenses	302	1,329
Net cash used in operating activities	(9,178)	(14,328)
Cash flows from investing activities:
Purchases of property and equipment	(35)	(28)
Net cash used in investing activities	(35)	(28)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock and convertible preferred stock liability, net of issuance costs	14,665	20,000
Proceeds from issuance of common stock		11
Proceeds from issuance of convertible note	100
Payment of deferred offering costs		(686)
Net cash provided by financing activities	14,765	19,325
Net increase in cash and cash equivalents, and restricted cash	5,552	4,969
Cash and cash equivalents, and restricted cash at beginning of period		11,257
Cash and cash equivalents, and restricted cash at end of period	5,552	16,226
Components of cash and cash equivalents, and restricted cash:
Cash and cash equivalents	5,528	16,188
Restricted cash	24	38
Cash and cash equivalents, and restricted cash at end of period	5,552	16,226
Supplemental disclosures of noncash investing and financing information:
Conversion of convertible note into preferred stock	$ 100
Deferred offering costs included in accounts payable and accrued expenses		$ 942

Organization and Basis of Presentation

9 Months Ended

Sep. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Basis of Presentation

1. Organization and Basis of Presentation

Description of Business

89bio, Inc. (“89bio” or the “Company”) is a clinical-stage biopharmaceutical company focused on the development and commercialization of innovative therapies for the treatment of liver and cardio-metabolic diseases. The Company’s lead product candidate, BIO89-100, a specifically engineered glycoPEGylated analog of fibroblast growth factor 21, is currently being developed for the treatment of nonalcoholic steatohepatitis.

89Bio Ltd. was incorporated in Israel in January 2018 and has two wholly owned subsidiaries: 89bio Management, Inc., located in San Francisco, California and UAB 89bio Lithuania, located in Vilnius, Lithuania. 89bio, Inc. was formed as a Delaware corporation on June 28, 2019, for the purpose of completing an initial public offering (“IPO”) and related transactions in order to carry on the business of 89Bio Ltd.

The Company completed an internal reorganization transaction in September 2019, pursuant to which 89Bio Ltd. became a wholly owned subsidiary of 89bio, Inc. (the “Reorganization”). As part of the Reorganization, all of the equity holders of 89Bio Ltd. exchanged 100% of the equity of 89Bio Ltd. for 100% of the equity of 89bio, Inc.

The Reorganization was considered a transaction between entities under common control. As a result, the accompanying condensed consolidated financial statements for periods prior to Reorganization have been adjusted to combine the previously separate entities for presentation purposes.

Reverse Stock Split

On October 24, 2019, 89bio, Inc.’s board of directors approved an amendment to the amended and restated certificate of incorporation of 89bio, Inc. to effect a 1-for-6.217 reverse split (“Reverse Split”) of shares of the common stock of 89bio, Inc. and a proportional adjustment to the conversion ratio of the convertible preferred stock, which was effected on October 25, 2019. The par value and authorized shares of common stock, and the par value, authorized and outstanding shares of the convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information for 89bio, Inc. included in the accompanying condensed consolidated financial statements has been adjusted to reflect the Reverse Split.

Initial Public Offering

On November 13, 2019, 89bio, Inc. completed the IPO, pursuant to which is issued and sold an aggregate of 6,100,390 shares of common stock (inclusive of 795,703 shares pursuant to the underwriters’ overallotment option) at the IPO price of $16.00 per share, resulting in net proceeds of $87.8 million after deducting underwriting discounts and commissions of $6.8 million and other offering expenses of $3.0 million. Upon the closing of the IPO, the Company’s outstanding convertible preferred stock automatically converted into shares of common stock (see Note 12).

Liquidity

The accompanying condensed consolidated financial statements have been prepared assuming the Company will continue as a going concern, which contemplates the realization of assets and liquidation of liabilities in the normal course of business. To date, the Company has not generated revenues from its activities and has incurred substantial operating losses. Management expects the Company to continue to generate substantial operating losses for the foreseeable future until it completes development of its products and seeks regulatory approvals to market such products. The Company had cash and cash equivalents of $16.2 million as of September 30, 2019. Management expects to continue to fund its operations primarily through utilization of its current financial resources and through additional raises of capital.

The Company intends to raise such capital through the issuance of additional equity financing and/or debt financing. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its products. The Company expects that its cash and cash equivalents as of September 30, 2019 together with the net proceeds received from its IPO (see Note 12), will be sufficient to fund operating expenses and capital expenditure requirements for a period of at least one year from the date the unaudited condensed consolidated financial statements are filed with the Securities and Exchange Commission (“SEC”).

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Unaudited Condensed Consolidated Financial Statements

The accompanying interim unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the Unites States (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial reporting.

The accompanying interim condensed consolidated financial statements are unaudited. The interim unaudited condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements as of December 31, 2018 and for the period from January 18, 2018 (inception) to December 31, 2018 and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and comprehensive loss, and cash flows. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 was derived from the audited financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated November 11, 2019 (“Prospectus”) that forms a part of the Company's Registration Statements on Form S-1 (File Nos. 333-234174 and 333-234617), as filed with the Securities and Exchange Commission (the “SEC”) pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended.

Use of Estimates

The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include but are not limited to the fair value of stock options, the convertible preferred stock liability and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.

Fair Value Measurements

Financial assets and liabilities are recorded at fair value on a recurring basis in the balance sheets. The carrying values of Company’s financial assets and liabilities, including cash and cash equivalents, restricted cash, prepaid and other current assets, accounts payable, and accrued expenses approximate to their fair value due to the short-term maturity of these instruments. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. Assets and liabilities recorded at fair value are categorized based upon the level of judgment associated with the inputs used to measure their fair value. Hierarchical levels are directly related to the amount of subjectivity with the inputs to the valuation of these assets or liabilities as follows:

Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable inputs for similar assets or liabilities. These include quoted prices for identical or similar assets or liabilities in active markets and quoted prices for identical or similar assets of liabilities in markets that are not active;

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Convertible Preferred Stock Liability

The freestanding instruments related to the commitment by the Series A convertible preferred stockholders to purchase and by the Company to sell its Series A convertible preferred stock in subsequent closings, contingent upon the achievement of certain developmental milestones and approval by the board of directors, at a fixed price per stock, are considered a liability (or an asset), measured at fair value as the shares underlying the rights contain liquidation preferences upon certain “deemed liquidation events” that are not solely within the Company’s control and which are considered in-substance contingent redemption features (refer to Note 7 for further discussion on the redemption rights of the convertible preferred stock). The instruments are subject to revaluation at each balance sheet date until settlement or extinguishment, with revaluations recognized as a component of other (income) expenses, net in the condensed consolidated statements of operations and comprehensive loss.

Deferred Offering Costs

The Company has deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the Company’s IPO. The deferred offering costs will be offset against the proceeds received upon the completion of the IPO. As of September 30, 2019, the Company recorded $1,628,000 of deferred offering costs on the condensed consolidated balance sheet. As of December 31, 2018, the Company did not have any deferred offering costs.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2014-09—Revenue from contracts with customers, to achieve a consistent application of revenue recognition, resulting in a single revenue model to be applied by reporting companies under U.S. GAAP. Under the new model, recognition of revenue occurs when a customer obtains control of the promised goods or services in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In addition, the standard requires that reporting companies disclose the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The standard is effective for public entities for fiscal years beginning after December 15, 2017 and is effective for nonpublic entities for fiscal years beginning after December 15, 2018. The standard is required to be applied retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying it recognized at the date of initial application. The Company adopted this standard on January 1, 2019, and as the Company has no revenues to date, the adoption of the standard did not have any impact on its consolidated financial statements.

In February 2016, the FASB issued ASU 2016-02—Leases, requiring the recognition of lease assets and liabilities on the balance sheet. The standard: (a) clarifies the definition of a lease; (b) requires a dual approach to lease classification similar to current lease classifications; and (c) causes lessees to recognize leases on the balance sheet as a lease liability with a corresponding right-of-use asset for leases with a lease-term of more than twelve months. The standard is effective for public entities for fiscal years beginning after December 15, 2018 and for nonpublic entities for fiscal years beginning after December 15, 2020. The Company is currently evaluating the impact of this standard and expects the adoption will result in an insignificant increase in the assets and liabilities on its consolidated balance sheet for operating leases.

In June 2018, the FASB issued ASU No. 2018-07 Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which is intended to reduce the cost and complexity and to improve financial reporting for nonemployee share based payment. The standard expands the scope of Topic 718, (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The standard is effective for public entities for fiscal years beginning after December 15, 2019 and nonpublic entities for fiscal years beginning after December 15, 2020. Early adoption is permitted but no earlier than a company’s adoption date of Topic 606. The Company early adopted this standard on January 1, 2019, and the impact of its adoption on the Company’s consolidated financial statements was not material.

Fair Value Measurements

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The fair value of the Company’s financial liabilities measured at fair value on a recurring basis by level within the fair value hierarchy are as follows (in thousands):

	September 30, 2019
	Level 1		Level 2		Level 3		Total
Liabilities:
Convertible preferred stock liability	$	—	$	—	$	16,016	$	16,016
Total financial liabilities	$	—	$	—	$	16,016	$	16,016

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities:
Convertible preferred stock liability	$	—	$	—	$	1,671	$	1,671
Total financial liabilities	$	—	$	—	$	1,671	$	1,671

The changes in the fair value of the Company’s Level 3 financial liabilities, which are measured on a recurring basis are as follows (in thousands):

	September 30,			December 31,
	2019			2018
Beginning balance	$	1,671		$	—
Recognition of convertible preferred stock liability upon issuance of convertible preferred stock		—			638
Revaluation of convertible preferred stock liability recorded in other (income) expense, net		21,018			979
Partial settlement of convertible preferred stock liability upon second closing		(1,860	)		54
Partial settlement of convertible preferred stock liability upon third closing		(4,813	)		—
Ending balance	$	16,016		$	1,671

The fair value of the Company’s convertible preferred stock liability is based on significant inputs not observed in the market, and thus represent a Level 3 measurement. Refer to Note 7 for further discussion on the convertible preferred stock liability.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Consolidated Balance Sheet Components

9 Months Ended

Sep. 30, 2019

Balance Sheet Related Disclosures [Abstract]

Consolidated Balance Sheet Components

4. Consolidated Balance Sheet Components

Accrued Expenses

Accrued expenses consist of the following (in thousands):

	September 30,		December 31,
	2019		2018
Accrued research and development expense	$	807	$	890
Accrued employee and related expenses		2,185		283
Total accrued expenses	$	2,992	$	1,173

Commitments and Contingencies

9 Months Ended

Sep. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5. Commitments and Contingencies

Asset Transfer and License Agreement with Teva Pharmaceutical Industries Ltd

In April 2018, the Company concurrently entered into two Asset Transfer and License Agreements (the “Teva Agreements”) with Teva Pharmaceutical Industries Ltd (“Teva”) under which it acquired certain patents and intellectual property relating to two programs: (1) Teva’s glycoPEGylated FGF21 program, including the compound TEV-47948 (BIO89-100), a glycoPEGylated long-acting FGF21 and (2) Teva’s development program of small molecule inhibitors of Fatty Acid Synthase. Pursuant to the Teva Agreements, the Company paid Teva an initial nonrefundable upfront payment of $6.0 million and the Company could be obligated to pay Teva up to $67.5 million under each program, for a total of $135.0 million, upon the achievement of certain clinical development and commercial milestones. In addition, the Company is obligated to pay Teva tiered royalties at percentages in the low-to-mid single-digits on worldwide net sales on all products containing the Teva compounds.

The Teva Agreements can be terminated (i) by the Company without cause, after the first anniversary of the effective date, upon 120 days’ written notice to Teva, (ii) by either party, if the other party materially breaches any of its obligations under the Agreements and fails to cure such breach within 60 days after receiving notice thereof, or (iii) by either party, if a bankruptcy petition is filed against the other party and is not dismissed within 60 days. In addition, Teva can also terminate the agreement related to the Company’s glycoPEGylated FGF21 program in the event the Company, or any of its affiliates or sublicensees, challenges any of the Teva patents licensed to the Company, and the challenge is not withdrawn within 30 days of written notice from Teva.

The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the total consideration transferred to Teva as research and development expense in the condensed consolidated statements of operations and comprehensive loss because the acquired technology represented in-process research and development and had no alternative future use.

Convertible Note

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Convertible Note

6. Convertible Note

In March 2018, the Company entered into a Convertible Loan Agreement (the “Convertible Note”) with a principal amount of $100,000 and a fixed interest rate of 8% per annum. The Convertible Note was automatically convertible into the Company’s next equity financing, or upon an earlier event of default. In April 2018, the entire amount outstanding converted into 100,000 shares of Series A convertible preferred stock upon the initial closing of the Series A financing.

Convertible Preferred Stock

9 Months Ended

Sep. 30, 2019

Temporary Equity Disclosure [Abstract]

Convertible Preferred Stock

7. Convertible Preferred Stock

In April 2018, the Company entered into the Series A Share Purchase Agreement (the “SPA”), pursuant to which the investors committed to invest an aggregate amount of up to $60.0 million for the issuance of shares of Series A convertible preferred stock at a price of $1.00 per share.

The initial closing occurred on April 16, 2018, and the Company issued 14,900,000 shares of Series A convertible preferred stock at a price per share of $1.00 for net cash proceeds of $14.7 million. The investors also committed to purchase 15,000,000 and 30,000,000 shares of Series A convertible preferred stock at a price of $1.00 per share in second and third closings, respectively, contingent upon the achievement by the Company of certain development milestones and approval by the board of directors.

The investors’ commitment to purchase and the Company’s commitment to sell shares of Series A convertible preferred stock represent a freestanding instrument accounted for at fair value and re-measured at each reporting date. The Company estimates the fair value of this commitment using the Black Scholes option pricing model. On the date of the initial closing, the Company recorded the commitments associated with the second and third closings of the Series A convertible preferred stock at a net value of $638,000. For the three months ended September 30, 2019 and 2018, the Company recorded an expense of $10.5 million and $398,000 , respectively, for the revaluation of the convertible preferred stock liability, within other (income) expenses, net in the condensed consolidated statements of operations and comprehensive loss. For the nine months ended September 30, 2019 and for the period from January 18, 2018 (inception) to September 30, 2018, the Company recorded an expense of $21.0 million and $755,000, respectively, for the revaluation of the convertible preferred stock liability, within other (income) expenses, net in the condensed consolidated statements of operations and comprehensive loss.

In December 2018, the Series A convertible preferred stockholders partially accelerated the second closing and the Company issued 9,000,000 shares of Series A convertible preferred stock at a price of $1.00 per share and received net proceeds of $9.0 million.

In June 2019, the Company and the Series A convertible preferred stockholders agreed to issue the remaining 6,000,000 shares of Series A convertible preferred stock at a price of $1.00 per share related to the second closing, and to partially accelerate 14,000,000 shares of Series A convertible preferred stock at a price of $1.00 per share related to the third closing. The shares were issued and the aggregate net proceeds of $20.0 million were received in June and July 2019.

In September 2019, the Company completed an internal reorganization transaction pursuant to which 89Bio Ltd. became a wholly owned subsidiary of 89bio, Inc. As part of the Reorganization, all outstanding convertible preferred shares of 89Bio Ltd. were exchanged into shares of convertible preferred stock of 89bio, Inc.

As of September 30, 2019, 16,000,000 shares of Series A convertible preferred stock were subject to issuance upon completion of remaining milestones or as the preferred stockholders elected to waive such milestones.

Convertible Preferred Stock

Convertible preferred stock consists of the following:

			Shares
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		60,000,000		44,000,000	$	49,746,000	$	44,000,000
Total		60,000,000		44,000,000	$	49,746,000	$	44,000,000

			Shares
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		60,000,000		24,000,000	$	23,073,000	$	24,000,000
Total		60,000,000		24,000,000	$	23,073,000	$	24,000,000

The Company’s articles of association do not provide redemption rights to the holders of the Series A convertible preferred stock. In the event of a liquidation event, all the funds and assets of the Company available for distribution among all the stockholders shall be distributed in the following order of preference: (a) the holders of the Series A convertible preferred stock shall be entitled to receive an amount per share equal to $1.00 per each Series A convertible preferred stock (less the amount of distributions actually received in any prior liquidation event, plus all declared but unpaid dividends) and (b) the remaining assets of the Company available for distribution to stockholders shall be distributed among the holders of common stock and to the holders of the Series A convertible preferred stock on an as-converted and pro rata basis.

Although the convertible preferred stock is not redeemable, in the event of certain “deemed liquidation events” that are not solely within the Company’s control (including merger, acquisition, or sale of all or substantially all of the Company’s assets), the holders of the convertible preferred stock would be entitled to preference amounts paid before distribution to other stockholders and hence effectively redeeming the preference amount. The convertible preferred stock is classified outside of stockholders’ deficit as a result of these in-substance contingent redemption rights.

As of September 30, 2019, and December 31, 2018, the Company did not adjust the carrying values of the convertible preferred stock to the deemed liquidation values of such shares since a liquidation event was not probable of occurring.

Upon the closing of the IPO on November 13, 2019, the Company’s outstanding convertible preferred stock automatically converted into 7,077,366 shares of common stock of 89bio, Inc. (see Note 12).

Common Stock

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

Common Stock

8. Common Stock

Pursuant to the Company’s amended articles of association filed in September 2019, the Company is authorized to issue a total of 72,882,353 shares of commons stock with a par value of $0.001 per share, 611,226 of which were issued and outstanding as of September 30, 2019.

In September 2019, the Company completed an internal reorganization transaction pursuant to which 89Bio Ltd. became a wholly owned subsidiary of 89bio, Inc. As part of the Reorganization, all outstanding ordinary shares of 89Bio Ltd. were exchanged into shares of common stock of 89bio, Inc.

Total common stock reserved for issuance is summarized as follows:

	September 30,		December 31,
	2019		2018
Series A convertible preferred stock outstanding, as converted		7,077,366		3,860,383
Options issued and outstanding		1,251,885		591,448
Shares available for future option grants		209,006		472,714
Total common stock reserved for issuance		8,538,257		4,924,545

Share-Based Compensation

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

9. Share-Based Compensation

Equity Incentive Plans

In 2018, the Company’s board of directors adopted the 89Bio Ltd. 2018 Equity Incentive Plan (the “2018 Plan”). In connection with the Reorganization, in September 2019, the Company’s board of directors approved the 2019 Equity Incentive Plan (the “2019 Plan”), which became effective on September 17, 2019. From and after the effective date of the 2019 Plan, the Company will no longer be making any future awards under the 2018 Plan. In addition, as part of the Reorganization, all outstanding options of 89Bio Ltd were exchanged for options to purchase common stock of 89bio, Inc. Option amounts for periods prior to the Reorganization give effect to this exchange. As of September 30, 2019, there were 209,006 shares of common stock reserved and available for issuance as future option grants under the 2019 Plan.

The Company recorded share-based compensation for the periods indicated as follows (in thousands):

							Period from
							January 18,
	Three Months		Three Months		Nine Months		2018
	Ended		Ended		Ended		(inception) to
	September 30,		September 30,		September 30,		September 30,
	2019		2018		2019		2018
Research and development	$	34	$	—	$	54	$	—
General and administrative		78		—		169		—
Total share-based compensation	$	112	$	—	$	223	$	—

The fair value of option awards granted for the periods indicated was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

			Period from
			January 18,
	Nine Months		2018
	Ended		(inception) to
	September 30,		September 30,
	2019		2018
Expected term (years)	5.88–6.11			—
Expected volatility	61.80-72.45%			—
Risk-free interest rate	1.64-2.60%			—
Expected dividend		—		—

The following table summarizes stock option activity under the 2018 Plan for the nine months ended September 30, 2019:

						Weighted
				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Options			Price		Term		Value
						(In years)		(In thousands)
Balance outstanding as of December 31, 2018		591,448		$	1.93		9.87	$	—
Granted		673,927			2.86
Exercised		—			—
Cancelled		(13,490	)		1.93
Balance outstanding as of September 30, 2019		1,251,885		$	2.43		9.45	$	10,036
Exercisable as of September 30, 2019		170,175		$	1.93		9.13	$	1,449

Net Loss Per Share

9 Months Ended

Sep. 30, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

10. Net Loss Per Share

The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect:

			Period from
			January 18,
	Nine Months		2018
	Ended		(inception) to
	September 30,		September 30,
	2019		2018
Convertible preferred stock, as converted		7,077,366		2,412,739
Stock options to purchase ordinary shares		1,251,885		—
Total		8,329,251		2,412,739

Related Party Transactions

9 Months Ended

Sep. 30, 2019

Related Party Transactions

11. Related Party Transactions

The Company incurred $90,000 and $6,000 in professional services expense related to certain members of the board of directors for the three months ended September 30, 2019 and 2018, respectively, The Company incurred $210,000 and $132,000 in professional services expense related to certain members of the board of directors for the nine months ended September 30, 2019 and for the period from January 18, 2018 (inception) to September 30, 2018 and, respectively. The related party liability balance was $4,200 and $23,000 as of September 30, 2019, and December 31, 2018, respectively.

Subsequent Events

9 Months Ended

Sep. 30, 2019

Subsequent Events [Abstract]

Subsequent Events

12. Subsequent Events

2019 Equity Incentive Plan

On October 24, 2019, 89bio, Inc.’s board of directors approved an increase the aggregate maximum number of shares of common stock that may be issued pursuant to stock awards under 2019 Plan to 2,844,193 shares of common stock. On January 1, 2020, and each January 1 thereafter until January 1, 2029, the shares issuable pursuant to stock awards under the 2019 Plan will automatically increase by 4% of the number of shares of the Company’s capital stock issued and outstanding on the immediately preceding December 31, or a lesser number of shares as determined by the board of directors.

Reverse Stock Split

On October 24, 2019, 89bio, Inc.’s board of directors approved the Reverse Split, which was effected on October 25, 2019 (see Note 1).

Series A Share Purchase Agreement

On October 25, 2019, the Company and the Series A preferred stockholders amended the Series A SPA, and the parties agreed that the Series A SPA would terminate upon consummation of the Company’s IPO. Accordingly, the 16,000,000 shares of Series A convertible preferred stock that were issuable upon completion of remaining milestones or as the preferred stockholders elected to waive such milestones, are no longer issuable under the Series SPA after the IPO.

Initial Public Offering

Upon the closing of the IPO, the Company’s outstanding convertible preferred stock automatically converted into 7,077,366 shares of common stock of 89bio, Inc. based on a proportional adjustment to the conversion ratio of the convertible preferred stock on a 1-for-6.217.

Lease Agreement

On December 9, 2019, the Company entered into a lease in San Francisco, California to obtain approximately 3,554 rentable square feet as its headquarters. The initial term of the lease is two years and the base rent for the first year will be $210,000, increasing 3% annually.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2019
Accounting Policies [Abstract]
Unaudited Condensed Consolidated Financial Statements	Unaudited Condensed Consolidated Financial Statements The accompanying interim unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the Unites States (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial reporting. The accompanying interim condensed consolidated financial statements are unaudited. The interim unaudited condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements as of December 31, 2018 and for the period from January 18, 2018 (inception) to December 31, 2018 and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and comprehensive loss, and cash flows. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 was derived from the audited financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated November 11, 2019 (“Prospectus”) that forms a part of the Company's Registration Statements on Form S-1 (File Nos. 333-234174 and 333-234617), as filed with the Securities and Exchange Commission (the “SEC”) pursuant to Rule 424 promulgated under the Securities Act of 1933, as amended.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying condensed consolidated financial statements include but are not limited to the fair value of stock options, the convertible preferred stock liability and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Fair Value Measurements	Fair Value Measurements Financial assets and liabilities are recorded at fair value on a recurring basis in the balance sheets. The carrying values of Company’s financial assets and liabilities, including cash and cash equivalents, restricted cash, prepaid and other current assets, accounts payable, and accrued expenses approximate to their fair value due to the short-term maturity of these instruments. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. Assets and liabilities recorded at fair value are categorized based upon the level of judgment associated with the inputs used to measure their fair value. Hierarchical levels are directly related to the amount of subjectivity with the inputs to the valuation of these assets or liabilities as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable inputs for similar assets or liabilities. These include quoted prices for identical or similar assets or liabilities in active markets and quoted prices for identical or similar assets of liabilities in markets that are not active; Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
Convertible Preferred Stock Liability	Convertible Preferred Stock Liability The freestanding instruments related to the commitment by the Series A convertible preferred stockholders to purchase and by the Company to sell its Series A convertible preferred stock in subsequent closings, contingent upon the achievement of certain developmental milestones and approval by the board of directors, at a fixed price per stock, are considered a liability (or an asset), measured at fair value as the shares underlying the rights contain liquidation preferences upon certain “deemed liquidation events” that are not solely within the Company’s control and which are considered in-substance contingent redemption features (refer to Note 7 for further discussion on the redemption rights of the convertible preferred stock). The instruments are subject to revaluation at each balance sheet date until settlement or extinguishment, with revaluations recognized as a component of other (income) expenses, net in the condensed consolidated statements of operations and comprehensive loss.
Deferred Offering Costs	Deferred Offering Costs The Company has deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the Company’s IPO. The deferred offering costs will be offset against the proceeds received upon the completion of the IPO. As of September 30, 2019, the Company recorded $1,628,000 of deferred offering costs on the condensed consolidated balance sheet. As of December 31, 2018, the Company did not have any deferred offering costs.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2014-09—Revenue from contracts with customers, to achieve a consistent application of revenue recognition, resulting in a single revenue model to be applied by reporting companies under U.S. GAAP. Under the new model, recognition of revenue occurs when a customer obtains control of the promised goods or services in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In addition, the standard requires that reporting companies disclose the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The standard is effective for public entities for fiscal years beginning after December 15, 2017 and is effective for nonpublic entities for fiscal years beginning after December 15, 2018. The standard is required to be applied retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying it recognized at the date of initial application. The Company adopted this standard on January 1, 2019, and as the Company has no revenues to date, the adoption of the standard did not have any impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02—Leases, requiring the recognition of lease assets and liabilities on the balance sheet. The standard: (a) clarifies the definition of a lease; (b) requires a dual approach to lease classification similar to current lease classifications; and (c) causes lessees to recognize leases on the balance sheet as a lease liability with a corresponding right-of-use asset for leases with a lease-term of more than twelve months. The standard is effective for public entities for fiscal years beginning after December 15, 2018 and for nonpublic entities for fiscal years beginning after December 15, 2020. The Company is currently evaluating the impact of this standard and expects the adoption will result in an insignificant increase in the assets and liabilities on its consolidated balance sheet for operating leases. In June 2018, the FASB issued ASU No. 2018-07 Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which is intended to reduce the cost and complexity and to improve financial reporting for nonemployee share based payment. The standard expands the scope of Topic 718, (which currently only includes share-based payments to employees) to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. The standard is effective for public entities for fiscal years beginning after December 15, 2019 and nonpublic entities for fiscal years beginning after December 15, 2020. Early adoption is permitted but no earlier than a company’s adoption date of Topic 606. The Company early adopted this standard on January 1, 2019, and the impact of its adoption on the Company’s consolidated financial statements was not material.

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Summary of Liabilities Measured at Fair Value on Recurring Basis

The fair value of the Company’s financial liabilities measured at fair value on a recurring basis by level within the fair value hierarchy are as follows (in thousands):

	September 30, 2019
	Level 1		Level 2		Level 3		Total
Liabilities:
Convertible preferred stock liability	$	—	$	—	$	16,016	$	16,016
Total financial liabilities	$	—	$	—	$	16,016	$	16,016

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities:
Convertible preferred stock liability	$	—	$	—	$	1,671	$	1,671
Total financial liabilities	$	—	$	—	$	1,671	$	1,671

Summary of Changes in Fair Value of Level 3 Liabilities

The changes in the fair value of the Company’s Level 3 financial liabilities, which are measured on a recurring basis are as follows (in thousands):

	September 30,			December 31,
	2019			2018
Beginning balance	$	1,671		$	—
Recognition of convertible preferred stock liability upon issuance of convertible preferred stock		—			638
Revaluation of convertible preferred stock liability recorded in other (income) expense, net		21,018			979
Partial settlement of convertible preferred stock liability upon second closing		(1,860	)		54
Partial settlement of convertible preferred stock liability upon third closing		(4,813	)		—
Ending balance	$	16,016		$	1,671

Consolidated Balance Sheet Components (Tables)

9 Months Ended

Sep. 30, 2019

Balance Sheet Related Disclosures [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):

	September 30,		December 31,
	2019		2018
Accrued research and development expense	$	807	$	890
Accrued employee and related expenses		2,185		283
Total accrued expenses	$	2,992	$	1,173

Convertible Preferred Stock (Tables)

9 Months Ended

Sep. 30, 2019

Temporary Equity Disclosure [Abstract]

Schedule of Convertible Preferred Stock

Convertible preferred stock consists of the following:

			Shares
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		60,000,000		44,000,000	$	49,746,000	$	44,000,000
Total		60,000,000		44,000,000	$	49,746,000	$	44,000,000

			Shares
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		60,000,000		24,000,000	$	23,073,000	$	24,000,000
Total		60,000,000		24,000,000	$	23,073,000	$	24,000,000

Common Stock (Tables)

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

Schedule of Common Stock Reserved for Issuance

Total common stock reserved for issuance is summarized as follows:

	September 30,		December 31,
	2019		2018
Series A convertible preferred stock outstanding, as converted		7,077,366		3,860,383
Options issued and outstanding		1,251,885		591,448
Shares available for future option grants		209,006		472,714
Total common stock reserved for issuance		8,538,257		4,924,545

Share-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Share-Based Compensation

The Company recorded share-based compensation for the periods indicated as follows (in thousands):

							Period from
							January 18,
	Three Months		Three Months		Nine Months		2018
	Ended		Ended		Ended		(inception) to
	September 30,		September 30,		September 30,		September 30,
	2019		2018		2019		2018
Research and development	$	34	$	—	$	54	$	—
General and administrative		78		—		169		—
Total share-based compensation	$	112	$	—	$	223	$	—

Summary of Estimated Fair Value of Option Awards Granted Using Black-Scholes Option-Pricing Model

The fair value of option awards granted for the periods indicated was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

			Period from
			January 18,
	Nine Months		2018
	Ended		(inception) to
	September 30,		September 30,
	2019		2018
Expected term (years)	5.88–6.11			—
Expected volatility	61.80-72.45%			—
Risk-free interest rate	1.64-2.60%			—
Expected dividend		—		—

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2018 Plan for the nine months ended September 30, 2019:

						Weighted
				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Options			Price		Term		Value
						(In years)		(In thousands)
Balance outstanding as of December 31, 2018		591,448		$	1.93		9.87	$	—
Granted		673,927			2.86
Exercised		—			—
Cancelled		(13,490	)		1.93
Balance outstanding as of September 30, 2019		1,251,885		$	2.43		9.45	$	10,036
Exercisable as of September 30, 2019		170,175		$	1.93		9.13	$	1,449

Net Loss Per Share (Tables)

9 Months Ended

Sep. 30, 2019

Earnings Per Share [Abstract]

Schedule of Outstanding Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss

			Period from
			January 18,
	Nine Months		2018
	Ended		(inception) to
	September 30,		September 30,
	2019		2018
Convertible preferred stock, as converted		7,077,366		2,412,739
Stock options to purchase ordinary shares		1,251,885		—
Total		8,329,251		2,412,739

Organization and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands			9 Months Ended
	Nov. 13, 2019 USD ($) $ / shares shares	Oct. 24, 2019	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)
Subsidiary Sale Of Stock [Line Items]
Date of incorporation			Jun. 28, 2019
Reorganization exchange of equity percentage of ordinary shares exchanged			100.00%
Reorganization exchange of equity percentage of common stock issued upon exchange			100.00%
Reverse stock split, description			On October 24, 2019, 89bio, Inc.’s board of directors approved the Reverse Split, which was effected on October 25, 2019
Proceeds from issuance of common stock			$ 11
Cash and cash equivalents			$ 16,188	$ 11,234	$ 5,528
Subsequent Event
Subsidiary Sale Of Stock [Line Items]
Reverse stock split conversion ratio	0.160849	0.160849
Reverse stock split, description		On October 24, 2019, 89bio, Inc.’s board of directors approved an amendment to the amended and restated certificate of incorporation of 89bio, Inc. to effect a 1-for-6.217 reverse split (“Reverse Split”) of shares of the common stock of 89bio, Inc. and a proportional adjustment to the conversion ratio of the convertible preferred stock, which was effected on October 25, 2019.
Subsequent Event \| IPO \| Common Stock
Subsidiary Sale Of Stock [Line Items]
Shares issued and sold \| shares	6,100,390
IPO price per share \| $ / shares	$ 16.00
Proceeds from issuance of common stock	$ 87,800
Payment of underwriting discounts, commissions	6,800
Other offering expenses	$ 3,000
Subsequent Event \| Overallotment Option \| Common Stock
Subsidiary Sale Of Stock [Line Items]
Shares issued and sold \| shares	795,703

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Deferred offering costs	$ 1,628,000	$ 0

Fair Value Measurements - Summary of Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	$ 16,016	$ 1,671
Convertible Preferred Stock Liability
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	16,016	1,671
Level 3
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	16,016	1,671
Level 3 \| Convertible Preferred Stock Liability
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial liabilities	$ 16,016	$ 1,671

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Liabilities (Details) - Recurring - Level 3 - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value Liabilities Measured on Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 1,671
Recognition of convertible preferred stock liability upon issuance of convertible preferred stock		$ 638
Revaluation of convertible preferred stock liability recorded in other (income) expense, net	21,018	979
Partial settlement of convertible preferred stock liability upon second closing	(1,860)	54
Partial settlement of convertible preferred stock liability upon third closing	(4,813)
Ending balance	$ 16,016	$ 1,671

Consolidated Balance Sheet Components - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued research and development expense	$ 807	$ 890
Accrued employee and related expenses	2,185	283
Total accrued expenses	$ 2,992	$ 1,173

Commitments and Contingencies - Additional Information (Details) - Teva Agreements - USD ($)	1 Months Ended	9 Months Ended
	Apr. 30, 2018	Sep. 30, 2019
Loss Contingencies [Line Items]
Nonrefundable upfront payment	$ 6,000,000
Payment upon achievement of certain clinical and commercial milestones	135,000,000
Asset transfer and license agreement, termination period after written notice		120 days
Asset transfer and license agreement, termination period notice of breach		60 days
Asset transfer and license agreement termination notice period if bankruptcy petition filed		60 days
Teva’s GlycoPEGylated FGF21 Program
Loss Contingencies [Line Items]
Payment upon achievement of certain clinical and commercial milestones	67,500,000
Teva’s Development Program
Loss Contingencies [Line Items]
Payment upon achievement of certain clinical and commercial milestones	$ 67,500,000

Convertible Note - Additional Information (Details) - USD ($)	1 Months Ended	9 Months Ended
	Apr. 30, 2018	Sep. 30, 2019	Mar. 31, 2018
Convertible Note
Debt Instrument [Line Items]
Debt instrument, principal amount			$ 100,000
Fixed interest rate per annum			8.00%
Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Convertible note outstanding amount converted into shares	100,000
Convertible note, conversion description		the entire amount outstanding converted into 100,000 shares of Series A convertible preferred stock upon the initial closing of the Series A financing.

Convertible Preferred Stock - Additional Information (Details) - USD ($)		1 Months Ended			3 Months Ended		8 Months Ended	9 Months Ended
	Apr. 16, 2018	Jul. 31, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019	Nov. 13, 2019	Mar. 30, 2019	Jun. 30, 2018	Apr. 30, 2018	Mar. 30, 2018	Jan. 18, 2018
Temporary Equity [Line Items]
Net cash proceeds							$ 14,665,000	$ 20,000,000
Common Stock \| Subsequent Event
Temporary Equity [Line Items]
Convertible preferred stock automatically converted into common stock									7,077,366
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Number of shares committed to purchase					16,000,000			16,000,000
Convertible preferred stock, liquidation preference per share					$ 1.00			$ 1.00
Convertible Preferred Shares
Temporary Equity [Line Items]
Convertible preferred shares, outstanding			42,826,389	24,000,000		15,000,000	15,000,000			24,000,000	15,000,000		0	0
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Per share price of issuance of shares												$ 1.00
Net cash proceeds		$ 20,000,000	$ 20,000,000
Net value of commitment to purchase and sell shares	$ 638,000
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock \| Other (Income) Expense, Net
Temporary Equity [Line Items]
Expense recorded for revaluation of convertible preferred stock liability					$ 10,500,000	$ 398,000	$ 755,000	$ 21,000,000
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock \| Initial Closing
Temporary Equity [Line Items]
Per share price of issuance of shares	$ 1.00
Shares issued	14,900,000
Net cash proceeds	$ 14,700,000
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock \| Second Closing
Temporary Equity [Line Items]
Per share price of issuance of shares	$ 1.00		$ 1.00	$ 1.00
Shares issued				9,000,000
Net cash proceeds				$ 9,000,000
Number of shares committed to purchase	15,000,000
Remaining shares issued			6,000,000
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock \| Third Closing
Temporary Equity [Line Items]
Per share price of issuance of shares	$ 1.00		$ 1.00
Shares issued			14,000,000
Number of shares committed to purchase	30,000,000
Series A Share Purchase Agreement \| Series A Convertible Preferred Stock \| Maximum
Temporary Equity [Line Items]
Total amount of preferred shares issuable by the entity												$ 60,000,000

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	60,000,000	60,000,000
Convertible preferred stock, issued and outstanding	44,000,000	24,000,000
Convertible preferred stock, carrying value	$ 49,746,000	$ 23,073,000
Convertible preferred stock, liquidation preference	$ 44,000,000	$ 24,000,000
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	60,000,000	60,000,000
Convertible preferred stock, issued and outstanding	44,000,000	24,000,000
Convertible preferred stock, carrying value	$ 49,746,000	$ 23,073,000
Convertible preferred stock, liquidation preference	$ 44,000,000	$ 24,000,000

Common Stock - Additional Information (Details)	Sep. 30, 2019 $ / shares shares
Class Of Stock [Line Items]
Common stock, shares authorized	72,882,353
Common stock, par value \| $ / shares	$ 0.001
Common stock, shares issued	611,226
Common stock, shares outstanding	611,226

Common Stock - Schedule of Common Stock Reserved for Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Total common stock reserved for issuance	8,538,257	4,924,545
Series A Convertible Preferred Stock Outstanding Reserved for Issuance
Class Of Stock [Line Items]
Total common stock reserved for issuance	7,077,366	3,860,383
Options Issued and Outstanding Reserved for Issuance
Class Of Stock [Line Items]
Total common stock reserved for issuance	1,251,885	591,448
Shares Available for Future Option Grants Reserved for Issuance
Class Of Stock [Line Items]
Total common stock reserved for issuance	209,006	472,714

Share-Based Compensation - Additional Information (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved and available for issuance as future option grants	8,538,257	4,924,545
2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved and available for issuance as future option grants	209,006

Share-Based Compensation - Summary of Share-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation	$ 112	$ 223
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation	34	54
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation	$ 78	$ 169

Share-Based Compensation - Summary of Estimated Fair Value of Option Awards Granted Using Black-Scholes Option-Pricing Model (Details)	9 Months Ended
	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	61.80%
Expected volatility, maximum	72.45%
Risk-free interest rate, minimum	1.64%
Risk-free interest rate, maximum	2.60%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 10 months 17 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 1 month 9 days

Share-Based Compensation - Summary of Stock Option Activity (Details) - 2018 Plan - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Number of Options
Outstanding, Beginning Balance	591,448
Granted	673,927
Cancelled	(13,490)
Outstanding, Ending Balance	1,251,885	591,448
Exercisable as of September 30, 2019	170,175
Weighted Average Exercise Price
Weighted Average Exercise Price Outstanding, Beginning Balance	$ 1.93
Weighted Average Exercise Price, Granted	2.86
Weighted Average Exercise Price, Cancelled	1.93
Weighted Average Exercise Price Outstanding, Ending Balance	2.43	$ 1.93
Weighted Average Exercise Price, Exercisable as of September 30, 2019	$ 1.93
Weighted Average Remaining Contractual Term (In years)
Weighted Average Remaining Contractual Term (In years), Outstanding	9 years 5 months 12 days	9 years 10 months 13 days
Weighted Average Remaining Contractual Term (In years), Exercisable as of September 30, 2019	9 years 1 month 17 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value Outstanding	$ 10,036
Aggregate Intrinsic Value, Exercisable as of September 30, 2019	$ 1,449

Net Loss Per Share - Schedule of Outstanding Potentially Dilutive Securities Excluded From Calculation of Diluted Net Loss (Details) - shares	8 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	2,412,739	8,329,251
Convertible Preferred Stock, as Converted
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	2,412,739	7,077,366
Stock Options to Purchase Ordinary Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share		1,251,885

Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		8 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Outstanding liability to related party	$ 4,200			$ 4,200	$ 23,000
Board of Directors
Related Party Transaction [Line Items]
Professional services expense	$ 90,000	$ 6,000	$ 132,000	$ 210,000

Subsequent Events - Additional Information (Details) $ / shares in Units, $ in Thousands				9 Months Ended
	Dec. 09, 2019 USD ($) ft²	Nov. 13, 2019 USD ($) $ / shares shares	Oct. 24, 2019 shares	Sep. 30, 2019 USD ($) shares
Subsequent Event [Line Items]
Reverse stock split, description				On October 24, 2019, 89bio, Inc.’s board of directors approved the Reverse Split, which was effected on October 25, 2019
Proceeds from issuance of common stock, net of underwriters discount and commissions \| $				$ 11
Series A Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred shares issuable				16,000,000
Subsequent Event
Subsequent Event [Line Items]
Reverse stock split, description			On October 24, 2019, 89bio, Inc.’s board of directors approved an amendment to the amended and restated certificate of incorporation of 89bio, Inc. to effect a 1-for-6.217 reverse split (“Reverse Split”) of shares of the common stock of 89bio, Inc. and a proportional adjustment to the conversion ratio of the convertible preferred stock, which was effected on October 25, 2019.
Reverse stock split date			Oct. 25, 2019
Reverse stock split conversion ratio		0.160849	0.160849
Rentable area \| ft²	3,554
Lease term	2 years
Initial rent \| $	$ 210,000
Annual increase lease payment	3.00%
Subsequent Event \| Common Stock
Subsequent Event [Line Items]
Convertible preferred stock automatically converted into common stock		7,077,366
Subsequent Event \| IPO \| Common Stock
Subsequent Event [Line Items]
Issuance of ordinary shares, shares		6,100,390
IPO price per share \| $ / shares		$ 16.00
Proceeds from issuance of common stock, net of underwriters discount and commissions \| $		$ 87,800
Payment of underwriting discounts, commissions \| $		6,800
Other offering expenses \| $		$ 3,000
Subsequent Event \| Overallotment Option \| Common Stock
Subsequent Event [Line Items]
Issuance of ordinary shares, shares		795,703
Subsequent Event \| Series A Share Purchase Agreement \| Series A Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred shares issuable			16,000,000
2019 Plan \| Subsequent Event
Subsequent Event [Line Items]
Number of common shares issuable under the plan (in shares)			2,844,193
Percentage of increase in number of shares of capital stock issued and outstanding			4.00%