Teuvo Hakkarainen, a member of the anti-immigration Finns Party, was fined 3 060 euros (about R48 000).
He kissed Veera Ruoho, a member of the Conservatives, at a cafe in the parliament building after attending parliament’s Christmas party in December 2017.
The court also ordered Hakkarainen to pay Ruoho 1,400 euros in damages.
Ruoho hailed the conviction and called the trial “necessary”.
“Things like this should not be silenced and should not be tolerated,” she told Finnish news agency STT.
Hakkarainen, who told the court he had only vague memories of the evening because he was inebriated, admitted having kissed Ruoho against her will.
The defence argued, however, that he had not acted intentionally.
Hakkarainen has 30 days to appeal.
According to daily Helsingin Sanomat, the head of the Finns Party, Jussi Halla-aho, is not planning disciplinary action against Hakkarainen, though he was given a warning.
Hakkarainen has made headlines in the past for racist and homophobic remarks. In January 2017, he was found guilty of inciting racial hatred.