"The foreign ministry of the Republic of Bulgaria declared persona non grata a diplomat from the Russian embassy in Sofia and gave him 72 hours to leave the country due to activities incompatible with his diplomatic status," the foreign ministry said in a statement.
Bulgarian prosecutors said that from 2017 the diplomat engaged in spying activities, collecting military information, including about the numbers of US troops deployed on Bulgarian territory during exercises
The aim was to transfer this information to Russian military intelligence, prosecutors said, adding that the diplomat also paid a Bulgarian official with access to classified information to provide him with the sensitive data.
The spying row has soured relations between the two former allies, which have maintained their close cultural, historical and economic ties even after the fall of the communist regime in 1989.
Neither the foreign ministry nor the prosecution revealed the rank of the diplomat but the Russian embassy in Sofia said in a social media post that the measure concerned their chief military, air and naval attache.
The embassy called the expulsion "groundless", adding that it "does not contribute to the development of dialogue between our countries in the military sphere, as well as the strengthening of stability in the Black Sea region."
Russia also said it reserved the right to take retaliatory measures.
Since 2008, Bulgaria has been home to a joint-use US military training ground in Novo Selo, in the east of the country, for between 2,500 and 5,000 troops.
Bulgaria has hosted a number of US-led and NATO exercises over the past years.
US troops also use two Bulgarian air bases, Bezmer and Graf Ignatievo, as well as a storage site near Aytos in the east.
The US embassy in Bulgaria issued a statement on Friday backing the expulsion announcement.
"We have in recent weeks and months seen too many examples of Russian officials carrying out aggressive actions, from espionage in Bulgaria to poisoning opponents both at home and abroad. Bulgaria is a strong NATO ally and partner and has an unalienable right to defend its sovereignty," it said.
The expulsion on Friday coincided with the wrapping up of a three-day a visit to Bulgaria of NATO Special Operations Commander, Lieutenant General Eric Wendt.