The classified case against Yury Soloshenko, 73, was brought by the federal security service (FSB) which accused him of trying to smuggle sensitive missile defence technology out of Russia.
"The court has sentenced him to six years in a strict penal colony," a spokesman for the Moscow City Court told AFP, confirming the espionage charge.
Reports have identified Soloshenko as a pensioner from Ukraine's eastern city of Poltava. Prison observer Zoya Svetova had said that investigators kept defence attorneys from working on the case against Soloshenko's will.
Soloshenko has been under arrest since he was detained in August 2014, at the height of the conflict in eastern Ukraine between pro-Russian separatists and Kiev's forces, when the Ukrainian government accused Moscow of sending tanks and troops across the border.
"Soloshenko was detained by FSB employees in August 2014 in Moscow when he was trying to illegally obtain secret components for S-300 (missile system)," said an FSB statement distributed by Russian news agencies.
Russia has seen an unprecedented number of espionage and treason cases reach trial over the past year, as the conflict in Ukraine has exacerbated already-strained relations with West.
