Kuldeep Singh Sengar, who was a legislator in India's most populous northern state of Uttar Pradesh, was convicted by the court in Delhi under the Protection of Children from Sexual Offences Act, lawyer Tanveer Ahmed Mir said. Sengar had pleaded not guilty.
Sentencing is expected later in the week.
The case was thrust into the spotlight last year when the accuser, who was a teenager in 2017 when she says she was raped, tried to kill herself, accusing the police of inaction.
In July, a truck crashed into the car of the woman who had accused Sengar of rape. Two of her relatives were killed in the crash and she was injured. Since then the family has been provided with security.
Having faced criticism for failing to act swiftly against the politician, Prime Minister Narendra Modi's BJP expelled him from the party in August.Public anger has been mounting in India over sexual violence against women and the lack of speedy justice for the majority of victims.