Ukraine’s Crimea was incorporated into Russia in 2014 following a referendum.
"Capt. II rank Leonid Parkhomenko was fulfilling tasks assigned to him by the Main Intelligence Department of the Ukrainian Defence Ministry.
“He provided classified information on the Black Sea Fleet’s activities to the Foreign Secret Service," FSB said in a statement.
Meanwhile, the FSB has initiated a criminal case against the officer on charges of high treason.
The Black Sea Fleet is a large operational strategic command of the Russian navy, based in various harbours of the Black Sea and the Sea of Azov with headquarters in Sevastopol.
Ukraine’s Crimea was incorporated into Russia in 2014 following a referendum, whose results were recognized by Moscow, but rejected by Ukraine and Western powers.