The accused's suicide has prompted outrage from the victim's family.
Ram Singh, one of six people on trial over the shocking attack, was found dead shortly before dawn after making a noose out of his clothing, according to officials at Delhi's top-security Tihar jail.
"He tied all his clothes together and used the ceiling grill and a wooden stool to hang himself," the prison's law officer Sunil Gupta said by phone.
A judicial magistrate began an investigation into the suicide, which will probe any security lapses, Gupta added.
But the father of the 23-year-old gang-rape victim said it was clear that authorities were negligent and that the family was denied justice.
"We don't understand how could the police fail to protect Ram Singh. They knew he was the prime accused in my daughter's case," said the father, who cannot be named for legal reasons.
"How could they let him choose the way he wanted to die? The police have failed and I wonder what will happen to the case now."
The victim's mother said she was shocked to hear of the suicide.
"I just wanted justice for my daughter. The main accused is dead. Maybe the guilt killed him?" she said
Singh was making regular appearances in a New Delhi court where he faced murder, rape and kidnapping charges over the crime that ignited street protests and soul-searching about endemic sex crime in India.
Four other men charged over the gang-rape face the death sentence if convicted in a special fast-track court, whose proceedings are subject to a media gagging order.
They have all pleaded not guilty.
A sixth 17-year-old suspect is being tried in a juveniles' court where he faces a maximum three-year imprisonment.
The victim, who was studying physiotherapy, died on December 29 at a hospital in Singapore, 13 days after the attack. As well as being repeatedly raped, she was also violated with a metal bar, leaving her with massive internal injuries.
'Drinking, abusing and getting into fights'
Singh, a widower whose brother Mukesh is also an accused, was the regular driver of the white private bus alleged to have been used for the December 16 crime, which was normally used to ferry school children.
Police say Singh and friends enjoyed a meal together and were drinking heavily before they decided to take the bus out for a night-time joyride, picking up passengers who mistook the vehicle for genuine public transport.
Another senior police officer investigating the case, who asked not to be named, insisted that the suicide would not impact on the ongoing trial.
"The [court] case will continue. There is no reason for the case to suffer," said the officer, who also confirmed that Singh killed himself at 5.15am.
Another police officer deployed at Tihar Jail said Singh's body was taken to the government-run Deen Dayal Upadhyay hospital and his brother Mukesh was informed.
"We tried to save him and even employed revival techniques but failed," he said.
Singh, originally from western Rajasthan state, lived in a New Delhi slum called Ravi Dass colony where neighbours described him and his brother as rowdy and heavy drinkers.
"They were always drinking, abusing and getting into fights with us," one neighbour, who works as a domestic servant, said. – AFP