| Document and Entity Information | Sep. 09, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 9, 2021 |
| Trading Symbol | RLYB |
| Entity Registrant Name | RALLYBIO CORPORATION |
| Entity Central Index Key | 0001739410 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-40693 |
| Entity Tax Identification Number | 85-1083789 |
| Entity Address, Address Line One | 234 Church Street |
| Entity Address, Address Line Two | Suite 1020 |
| Entity Address, City or Town | New Haven |
| Entity Address, State or Province | CT |
| Entity Address, Postal Zip Code | 06510 |
| City Area Code | 203 |
| Local Phone Number | 859-3820 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
