"The court proceedings took place on March 17. Tamošaitis was sentenced to 12 years," Motuzas said in a telephone interview on Tuesday.
Tamošaitis' lawyer intends to appeal against the ruling, Motuzas has added.
A member of the Lithuanian Embassy's consular staff was present at the hearing held behind closed doors.
"The judge just read the defendant's first name, last name, place of residence and the sentence of 12 years. The case materials were not read, as they are marked as classified," said the ambassador.
Tamošaitis was detained by the Russian Federal Security Bureau (FSB) in Moscow in May 2015. Moscow then said that the Lithuanian citizen had admitted to working for Lithuania's military intelligence.
Tamošaitis was detained three weeks after a Russian citizen had been arrested in Lithuania on suspicions of spying for the FSB.