Ram Singh, who drove the bus in which the fatal rape took place on December 16, appeared to have hanged himself from a metal grille with a rope made from his clothes, a spokesman for Tihar Jail, Sunil Gupta, said in an interview.
Singh was one of five men, and one teenager, including his brother Mukesh, charged with rape and murder in the death of a 23-year old physiotherapy student. The case sparked widespread protests in India and a push to increase safety for women.
Singh was the first of the accused to provide details of the attack to the police. His confession helped them track the four other men and the juvenile who have been arrested in the case.
Singh shared a jail cell with other inmates, Gupta said, and it was unclear how he may have hanged himself without attracting their attention.
"I suspect there is foul play," said V.K. Anand, the lawyer representing Singh in the case. "There were no circumstances for committing suicide. His mental state was stable, the trial was going well, he was meeting with his family. I can't understand why he would commit suicide."
"This is a high-profile prisoner, and he was under special protection," Anand added. "He was never left alone. How could this happen?"
Gupta said an inquiry had begun.