The court said Asumal Harpalani, known to his followers as Asaram Bapu, raped a 16-year-old girl at an ashram in the western city of Jodhpur in 2013 on the pretext of exorcising her evil spirits. His attorneys said they are deciding whether to appeal the verdict.
With his beard and matching white robes, Harpalani, 77, preached celibacy and reportedly had about 20 million followers in India and assets worth $750 million.
His trial took place inside the Jodhpur prison, where he has been held since 2013, out of fear for how his followers might react to the verdict.
Two of the guru's aides also were convicted, while two others were acquitted.
Figures such as Harpalani are hugely popular in India, where religious worship over the past two decades has been increasingly dominated by television gurus whose sermons are beamed into millions of living rooms. Many of them also amass support because of their social welfare activities - notably setting up schools and hospitals that outperform poorly funded government institutions.
Recent scandals have led to some introspection among the devout, including attempts to distinguish "fake gurus" from "real" ones.
Many still express faith in Harpalani. His followers insisted in television interviews that the charges against him were made up by his opponents.
The victim's father, who cannot be named to protect his daughter's identity, told the ANI news agency that he was pleased with the verdict. "Asaram is convicted," he said. "We have got justice. I want to thank everyone who supported us in this fight."
Harpalani's attorneys said they were reviewing the judgment and trying to decide whether to appeal. They said the victim had misrepresented her age so that the guru would receive the toughest possible sentence.
"The girl says and the parents are saying that she was less than 18, but there's more than enough evidence which has come on the record to say that she was more than 18, so the age was wrongly given," Harpalani's attorney Vikas Pahwa told NDTV.
"Today some are hailing this as a victory of our criminal justice system, but at every step in this case, the criminal justice system of this country failed the victim and her family," he said.
Born into a poor family in what is now Pakistan, Harpalani built his spiritual empire from the ground up. Local reports suggest that he spent his childhood doing odd jobs - selling tea, transporting pilgrims on his pony cart and bootlegging - before he claimed to be an ascetic and set up his ashrams in the 1970s.
With the help of local politicians and believers, Harpalani soon oversaw a vast empire of about 400 ashrams and businesses selling various herbal products and publishing dozens of books of his teachings.
Since his arrest, Harpalani had been denied bail about a dozen times. In images from his court appearances over the past five years, he looks increasingly weak and fatigued.
The guru's fortunes may not have hit their nadir, though. In a separate case, Harpalani and his son are charged with raping two sisters in the western town of Surat. He also has been accused of killing four boys during black magic rituals and acquiring land illegally.
A number of witnesses testifying against him have been attacked - and some killed - although it is not clear whether the guru was involved in those attacks.
Harpalani's rape conviction is the second of its kind in recent months. In August, Gurmeet Ram Rahim Singh - a guru known for his sparkling outfits and his appearances in low-budget self-produced movies - was convicted of sexually assaulting two female followers. The conviction triggered riots by his devotees in which 39 people died.
Yashwant Jain, a member of the National Commission for Protection of Child Rights, applauded the victim's family for waging the legal battle against Harpalani. "We welcome the family who fought this so strongly and without whom none of this would have come out," he said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)