There were no external injury marks on his body, the doctor told the media on the condition of anonymity.
A panel of three doctors conducted Ram Singh's post-mortem at the All India Institute of Medical Sciences (AIIMS) before the body was handed over to his family.
An AIIMS source said Sanjeev Lalwani and D.N. Bhardwaj were among the doctors on the panel. The autopsy started at 10.45 a.m. and took about two hours.
According to AIIMS, the autopsy report will be given to police or the judiciary "within a couple of days".
Ram Singh reportedly committed suicide in Tihar Jail Monday morning. He was one of the six accused in the rape of a 23-year-old woman in a moving bus last year. Ram Singh was driving the bus.
He was arrested Dec 17, a day after the brutal gang-rape ignited protests all across India. A magisterial inquiry has been ordered into Ram Singh's death. His family says he was murdered.