Aleksandr Averin, co-chairman of the unregistered opposition Other Russia coalition, has been sentenced to three years in prison on charges of attempted weapons smuggling.
A court in the Rostov region bordering Ukraine handed down its ruling on December 7.
Averin was arrested in April while crossing the border between Russia and part of Ukraine's Luhansk region, which is controlled by pro-Russian separatists, after border guards found a pistol in his possession.
Other Russia, which Averin co-founded in 2006, has insisted that the pistol was planted and only Averin's thumbprint was found on the pistol.
The party has called Averin's arrest a provocation by Russian security officials.
Averin was one of the organizers of a series of opposition protests in 2006, during which he was repeatedly detained by security authorities.