Andrei Dumenkov was caught gathering information about prototype missile technology, the FSB said.
"Dumenkov was found guilty of treason in the form of espionage," the security body said. "He was sentenced to 12 years in prison, the term to be served in a high-security penal colony."
The FSB was tipped off about Dumenkov, who in 2004 made inquiries about classified information and offered to pay for it.
He was detained at a Moscow railroad station August 3, 2005, with classified military files on him.
The decision to arrest Dumenkov was prompted by reports he had completed formalities to emigrate to Germany. Dumenkov's contacts with German agents were proven, the FSB said.
During the investigation, Dumenkov confessed to spying.
