v3.26.1
Cover
 3 Months Ended
Mar. 31, 2026
Document Information [Line Items]  
Document Type S-4
Entity Registrant Name RALLYBIO CORPORATION
Entity Incorporation, State or Country Code DE
Entity Tax Identification Number 85-1083789
Entity Address, Address Line One PO Box no. 325
Entity Address, City or Town East Berlin
Entity Address, State or Province CT
Entity Address, Postal Zip Code 06023
City Area Code 203
Local Phone Number 859-3820
Entity Filer Category Non-accelerated Filer
Entity Small Business true
Entity Emerging Growth Company true
Entity Ex Transition Period false
Entity Central Index Key 0001739410
Amendment Flag false
Entity Primary SIC Number 2834
Business Contact [Member]  
Document Information [Line Items]  
Entity Address, Address Line One PO Box no. 325
Entity Address, City or Town East Berlin
Entity Address, State or Province CT
Entity Address, Postal Zip Code 06023
City Area Code 203
Local Phone Number 859-3820
Contact Personnel Name Stephen Uden, M.D.