Ram Singh, 33, was one of five men being held in the notorious Dehli rape and murder case was found hanged and dead in his prison cell.
Ram Singh was one of the suspects in the the notorious rape and murder of a female student in India's capital, Delhi.
The 23-year-old rape victim, who was not named for legal reasons, was with a male friend when she was attacked on a bus and thrown from the vehicle.
She died in a Singapore hospital on 29 December from massive internal injuries.
Tihar jail spokesman Sunil Gupta told the BBC that Ram Singh appeared to have hanged himself with an improvised rope made from a blanket at about 05:00 local time on Monday (23:30 GMT Sunday).
The spokesman denied Indian media reports that Ram Singh had been on suicide watch, and said three people were sharing his prison cell.