Josef S. was convicted of breach of the peace, attempted aggravated assault and serious damage to property by a court in the Austrian capital on Tuesday.
The 23-year-old received a one-year suspended prison sentence, four months of which he must spend in prison, and a three-year probation period.
Josef was alleged to have smashed windows and thrown stones and other objects at police officers during protests against the student fraternity ball, organized by the far-right Freedom Party of Austria (FPÖ).
The ball in January was attended by right-wing groups from across Europe.
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,” he said.
The science student 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.
Judges at the Vienna Regional Criminal Court said Josef, believed to be a member of the extreme left-wing Black Blok (Schwarze Block), was clearly a ringleader in the violent protests, in which shop windows were smashed and police officers attacked.
'Lack of evidence'
But many commentators said that there had been a lack of conclusive evidence.
Two photographers and one 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.
And voice analysis of the recordings, made by a policeman on his mobile phone, proved that the recordings were not of Josef.
Right-wing groups on Twitter greeted the verdict as "positive".
But 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".
The charge was based almost solely on information provided by a policeman who attended the riots in a civil reconnaissance capacity.
Despite contradictions between his statements and those of other policemen present that evening, the judge dismissed the discrepancies as "explainable errors".
In response to the fact the accused did not appear in any of the prolific video or photographic documentation of the riots, the judge stated: "It is a good thing we do not live in a police state where everything is recorded".
In his closing statement, the prosecutor painted an image of "a left-wing riot tourist who had not travelled to Vienna to demonstrate against right-wing extremists, but rather to ravage the city with a group of anarchists".
Heinz Patzelt, from Amnesty International Austria, later criticized the prosecutor's attack as "highly problematic".
Spiegel Online was also disparaging of the trial process.
"The process and verdict show three things," it wrote. "How unreliable witness evidence can be. How little one needs to do in Austria to be locked up and convicted of serious offences. And how great the prejudice is in the Austrian police and judicial system against left of centre activists."
Josef and his lawyer will decide in the coming days whether to appeal the verdict.