The 23-year-old from the German university town of Jena, near Leipzig, was sentenced to 12 months imprisonment.
He has already spent four months in pre-trial detention following his arrest on the night of the ball, which is linked to far-right politics.
The remaining eight months of his sentence have been declared non-custodial.
Judge Thomas Spreitzer at Vienna's 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.
The process leading to the verdict has drawn sharp criticism from many, including German media and the Secretary General of Amnesty International Austria, Heinz Patzelt.
'Left-wing riot tourist'
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, 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."
The FPÖ and the conservative People Party's (ÖVP) Vienna branch commented positively on the verdict.
The Greens and the Social Democrats' youth organisation urged for a debate about the law for breaching the peace, which is normally not used in trials any more.
Josef S's parents were both present during the proceedings.
His mother was visibly shaken by the wording of the prosecutor's closing argument, calling it "defamatory" and demanding a public apology.
His father also declared his son had been sentenced unjustly, however he said he was "pleased that the result of the judgement was that our son is free."
Josef S. and his defenders will decide in the coming days whether to appeal the verdict.