He was convicted of breach of the peace, attempted aggravated assault and serious damage to property.
He received a one-year suspended prison sentence, four months of which he must spend in prison, and a three year probation period.
Josef S. was alleged to have smashed windows and thrown stones and other objects at police officers during protests against the FPÖ's student fraternity ball.
His lawyer, Clemens Breitlahner, said Josef was "at the demonstrations, but was a peaceful participant and had nothing to do with these crimes. We believe there has been some confusion and our client was arrested in error."
Josef S has already spent six months in prison, in pre-trial detention. That time has been deducted from his sentence, meaning that he can now go home.
However, the verdict unleashed a storm of protest on Twitter, with one user saying "the right to demonstrate in Vienna has been abolished until further notice".
Right-wing groups on Twitter greeted the verdict as "positive".
Many commentators said that there had been a lack of conclusive evidence.
Two photographers and one ORF cameraman provided witness testimony on Monday.
Despite being in the middle of the action, none had seen the accused igniting a smoke bomb or engaging in violent behaviour.
One photographer testified that the accused was not to be seen in any of the 700 photos he took during the violence.
Video recordings submitted as incriminating evidence on the first day of the trial also showed no connection to the defendant.
Voice analysis of the recordings, made by a policeman on his private mobile phone, proved that the recordings were not of Josef S.