Retired Russian navy officer Leonid Parkhomenko has been sentenced to 14 years in prison after a military tribunal in the southern city of Rostov-on-Don found him guilty of passing classified documents to Ukrainian intelligence.
The North Caucasus Regional Military Court on December 11 also stripped Parkhomenko of his military rank.
Parkhomenko was detained in 2016 in the port of Sevastopol on Ukraine's Crimea Peninsula that was annexed by Russia in 2014.
He used to serve as an officer in Russia's naval fleet based in Sevastopol.
Investigators say Parkhomenko collected classified information about Russia's Black Sea Fleet and handed it to Ukraine in return for money between 2003-05.
The Federal Security Service (FSB) said Parkhomenko was receiving orders from the Ukrainian Defense Ministry.
After annexing Crimea in March 2014, Russia has provided military, political, and economic support to separatists in eastern Ukraine.
The conflict in the eastern part of Ukraine has claimed more than 10,300 lives since April 2014.
In November, the tension in the region escalated when Russia seized three Ukrainian Navy ships and arrested 24 sailors in the Kerch Strait that links the Black Sea with the Sea of Azov.