Vinay Sharma, 20, was admitted to the hospital May 4, two days after the assault, his lawyer, A.P. Singh, said.
"He is in a critical condition," Singh said. "He has had blood vomiting and chest pains."
Earlier this week, Singh requested a jail transfer for Sharma once he is released from the hospital.
Hospital staff declined to confirm Sharma's condition, and the jail's director said the allegations are false.
Sharma, three other men and a juvenile allegedly gang-raped a 23-year-old female student on Dec. 16.
Another defendant the case, Ram Singh, was found hanging in his cell inside Tihar Jail on March 11.
Jail authorities said it was a suicide, but Ram Singh's family insisted other inmates killed him.
The government has ordered an inquiry into Ram Singh's death.