Vinay Sharma, one of the five adult men convicted for their role in the infamous December 16 gangrape, tried to commit suicide on Wednesday night.
He first took excessive painkillers and thereafter tried to hang himself with a towel in Tihar jail. He was caught in the act by a Tamil Nadu special police personnel on duty who stopped him.
He has been admitted to the Deen Dayal Upadhyay Hospital in Delhi where his condition is critical.
Sharma is suffering from depression and was on medication, sources said.
Earlier in 2015, Vinay had claimed that he was beaten up by other inmates at the Tihar Jail. He had demanded extra security in jail after the attack.
He is not the first convict in the case to have attempted suicide. Ram Singh, the driver of the bus in which the medical student was brutally raped and tortured, was found hanging in his cell in Tihar in March 2013. Jail authorities had said he had committed suicide, but his family had then alleged that he had been murdered.
Six men, one of them a juvenile, were found guilty of raping and murdering a girl in a moving bus in 2012. She was flown to Singapore for treatment but had succumbed to her injuries after fifteen days.
Four convicts - Akshay Thakur, Vinay Sharma, Mukesh Singh and Pawan Gupta were sentenced to death in the case. The juvenile accused in the case was convicted and sentenced to a maximum of three years in a reformation home. He was released from the observation home in December last year.