Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation (Policies)	9 Months Ended
Sep. 30, 2021
Accounting Policies [Abstract]
IPO and Reorganization	IPO and Reorganization — As discussed in Note 1, in connection with the Reorganization, all of Rallybio Holdings' direct subsidiaries, other than Rallybio Corporation, became direct subsidiaries of the Company prior to the Liquidation of Rallybio Holdings. Prior to the completion of the Company's IPO, Rallybio Holdings liquidated and unitholders of Rallybio Holdings became the holders of common stock of Rallybio Corporation. Subsequent to the Reorganization and the Liquidation of Rallybio Holdings the condensed consolidated financial statements are reported from Rallybio Corporation. The capital structure of Rallybio Holdings prior to the Liquidation consisted of four classes of membership units: common units; Series A-1 preferred units; Series A-2 preferred units; Series B preferred units; and incentive common units ("incentive units"). The number of shares of common stock that the holders of each class of units received in the Liquidation was based on the value of Rallybio Holdings prior to the Liquidation, determined by reference to the initial public offering price per share of $13.00. The common stock of the Company was allocated to the holders of existing units in Rallybio Holdings as follows:  holders of 33,478,255 Series A-1 preferred units and Series A-2 preferred units of Rallybio Holdings received an aggregate of 5,257,590 shares of common stock of the Company;  holders of 104,442,965 Series B preferred units of Rallybio Holdings received an aggregate of 16,402,235 shares of common stock of the Company;  holders of 4,500,000 common units and 1,200,000 of performance based incentive common units of Rallybio Holdings received an aggregate of 706,701 and 188,454, respectively, in shares of common stock of the Company. Shares of common stock issued in respect of unvested common units and performance based incentive common units as of the Liquidation are shares of restricted common stock and subject to vesting in accordance with the vesting schedule applicable to such units; and  holders of 20,869,704 incentive units in Rallybio Holdings received an aggregate of 2,444,990 shares of common stock of the Company. Shares of common stock issued in respect of unvested incentive units as of the Liquidation are shares of restricted common stock and continue to be subject to vesting in accordance with the vesting schedule applicable to such incentive units. As a result of the Liquidation, the holders of units in Rallybio Holdings collectively were issued an aggregate of 24,999,970 shares of common stock of the Company prior to the completion of the IPO. The Reorganization and subsequent Liquidation resulted in a change in reporting entity as described in ASC 250. In accordance with the guidance applicable to these circumstances, the equity structure has been adjusted in all comparative periods up to the Liquidation to reflect the number of shares of the Company’s common stock, issued to Rallybio Holdings unitholders' in connection with the Liquidation. As such, historical Rallybio Holdings convertible redeemable preferred units, common units, and incentive units have been retroactively adjusted in these financial statements to shares and earnings per share in accordance with the ratio of common shares received by each membership unit class. Rallybio Holdings' convertible redeemable preferred units previously classified as mezzanine equity have been retroactively adjusted in these financial statements, converted into common stock, and reclassified to permanent as a result of the retrospective application of the Liquidation and change in reporting entity.
Unaudited Financial Information	Unaudited Financial Information— The unaudited condensed consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP"), and pursuant to the rules and regulations of the Securities and Exchange Commission (the "SEC"). In the opinion of the Company, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the financial position and results of operations for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period. The Company reviews new accounting standards as issued. As of September 30, 2021 the Company has not identified any new standards that it believes will have a significant impact on the financial statements of the Company. However, the Company is still evaluating the impact of adopting ASU 2016-02, Leases on its condensed consolidated financial statements. There were no changes to significant accounting policies of the Company during the nine months ended September 30, 2021 other than noted below. These accompanying unaudited condensed consolidated financial statements and notes should be read in conjunction with the audited consolidated financial statements for the year ended December 31, 2020 and notes thereto contained in the Company’s final prospectus for its IPO filed with the SEC on July 30, 2021 pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “IPO Prospectus”)
Deferred Offering Costs	Deferred Offering Costs— The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such equity financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders' equity as a reduction of additional paid-in-capital generated as a result of the offering. Should the planned equity financing no longer be considered probable of being consummated, the offering costs are expensed immediately as a charge to operating expense. Upon completion of the Company's IPO in the third quarter of 2021, $3.2 million of deferred offering costs were charged against additional paid-in-capital. As a result, as of September 30, 2021, the Company did not have any deferred offering costs.

Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation (Policies)

9 Months Ended

Sep. 30, 2021

IPO and Reorganization — As discussed in Note 1, in connection with the Reorganization, all of Rallybio Holdings' direct subsidiaries, other than Rallybio Corporation, became direct subsidiaries of the Company prior to the Liquidation of Rallybio Holdings. Prior to the completion of the Company's IPO, Rallybio Holdings liquidated and unitholders of Rallybio Holdings became the holders of common stock of Rallybio Corporation. Subsequent to the Reorganization and the Liquidation of Rallybio Holdings the condensed consolidated financial statements are reported from Rallybio Corporation.

The capital structure of Rallybio Holdings prior to the Liquidation consisted of four classes of membership units: common units; Series A-1 preferred units; Series A-2 preferred units; Series B preferred units; and incentive common units ("incentive units"). The number of shares of common stock that the holders of each class of units received in the Liquidation was based on the value of Rallybio Holdings prior to the Liquidation, determined by reference to the initial public offering price per share of $13.00. The common stock of the Company was allocated to the holders of existing units in Rallybio Holdings as follows:



holders of 33,478,255 Series A-1 preferred units and Series A-2 preferred units of Rallybio Holdings received an aggregate of 5,257,590 shares of common stock of the Company;



holders of 104,442,965 Series B preferred units of Rallybio Holdings received an aggregate of 16,402,235 shares of common stock of the Company;



holders of 4,500,000 common units and 1,200,000 of performance based incentive common units of Rallybio Holdings received an aggregate of 706,701 and 188,454, respectively, in shares of common stock of the Company. Shares of common stock issued in respect of unvested common units and performance based incentive common units as of the Liquidation are shares of restricted common stock and subject to vesting in accordance with the vesting schedule applicable to such units; and



holders of 20,869,704 incentive units in Rallybio Holdings received an aggregate of 2,444,990 shares of common stock of the Company. Shares of common stock issued in respect of unvested incentive units as of the Liquidation are shares of restricted common stock and continue to be subject to vesting in accordance with the vesting schedule applicable to such incentive units.

As a result of the Liquidation, the holders of units in Rallybio Holdings collectively were issued an aggregate of 24,999,970 shares of common stock of the Company prior to the completion of the IPO.

The Reorganization and subsequent Liquidation resulted in a change in reporting entity as described in ASC 250. In accordance with the guidance applicable to these circumstances, the equity structure has been adjusted in all comparative periods up to the Liquidation to reflect the number of shares of the Company’s common stock, issued to Rallybio Holdings unitholders' in connection with the Liquidation. As such, historical Rallybio Holdings convertible redeemable preferred units, common units, and incentive units have been retroactively adjusted in these financial statements to shares and earnings per share in accordance with the ratio of common shares received by each membership unit class. Rallybio Holdings' convertible redeemable preferred units previously classified as mezzanine equity have been retroactively adjusted in these financial statements, converted into common stock, and reclassified to permanent as a result of the retrospective application of the Liquidation and change in reporting entity.

Unaudited Financial Information

Unaudited Financial Information— The unaudited condensed consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP"), and pursuant to the rules and regulations of the Securities and Exchange Commission (the "SEC"). In the opinion of the Company, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair presentation of the financial position and results of operations for the reported interim periods. The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The results of operations for interim periods are not necessarily indicative of results to be expected for the full year or any other interim period.

The Company reviews new accounting standards as issued. As of September 30, 2021 the Company has not identified any new standards that it believes will have a significant impact on the financial statements of the Company. However, the Company is still evaluating the impact of adopting ASU 2016-02, Leases on its condensed consolidated financial statements.

There were no changes to significant accounting policies of the Company during the nine months ended September 30, 2021 other than noted below.

These accompanying unaudited condensed consolidated financial statements and notes should be read in conjunction with the audited consolidated financial statements for the year ended December 31, 2020 and notes thereto contained in the Company’s final prospectus for its IPO filed with the SEC on July 30, 2021 pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “IPO Prospectus”)

Deferred Offering Costs

Deferred Offering Costs— The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such equity financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders' equity as a reduction of additional paid-in-capital generated as a result of the offering. Should the planned equity financing no longer

be considered probable of being consummated, the offering costs are expensed immediately as a charge to operating expense. Upon completion of the Company's IPO in the third quarter of 2021, $3.2 million of deferred offering costs were charged against additional paid-in-capital. As a result, as of September 30, 2021, the Company did not have any deferred offering costs.