It was the first verdict under legislation pushed through by authoritarian leader Alexander Lukashenko before the March presidential election.
Sergei Lyashkevich, who headed opposition presidential candidate Alexander Milinkevich's campaign office in the Western town of Shchuchin, was found guilty by a court there, according to the defendant and the human rights group Vyasna.
The verdict was the first issued under an amendment to the criminal code that provides for sentences of up to two years in prison for anyone convicted of financing or "teaching and preparing individuals to take part in mass disorder."
The amendment was part of a package of legislation approved by the loyal parliament in December, before the March 19 presidential election in which Lukashenko won a third term in a vote the opposition and Western observers and governments called illegitimate, the AP reports.
After days of peaceful opposition protests following the election, police broke up a tent camp on the central square in the capital, Minsk, and beat demonstrators participating in a protest march. Hundreds of opposition supporters were detained during the campaign and after the election.