Michail Marinich, a former presidential candidate, was found guilty of stealing computers, faxes and mobile telephones on Friday - equipment which the US embassy had provided to his non-governmental organisation Business Initiative.
Washington says it has no claim against Marinich and called last week for his release, denouncing the charges as politically motivated.
The 64-year-old politician was cleared of a second charge of illegally possessing firearms.
Marinich told the court after the verdict was read out: "I have faith that the leaders of the G8 will create an international committee that will appreciate the arbitrary goings-on in Belarus."
Nine month trial
His lawyer Vera Stremkovskaya insisted throughout the trial that his conviction was based on false evidence.
One of the leaders of the opposition movement For a Better Life in Belarus, Marinich was arrested in April on suspicion of firearms possession.
He has accused the authorities in this isolated former Soviet republic of persecuting him and has asked for the support of EU countries.
A former mayor of Minsk, former minister of external economic affairs and former ambassador to Estonia, Finland and Latvia, Marinich left his diplomatic career in 2001 in order to challenge President Alexander Lukashenko.