Bulgarian PM Kiril Petkov. Photo courtesy of his Twitter account (@KirilPetkov)
"Bulgaria is going to expel 70 Russian diplomats... Our services identified them as people who worked against our interests," Prime Minister Kiril Petkov told reporters.
"Everyone who works against the interests of Bulgaria will be called upon to return to their country. We expect a full 70-seat plane to fly back to Moscow," Petkov added.
Those identified to be a "threat to national security" must leave by July 3, the foreign ministry said.
Multiple European countries have expelled Russian diplomats following Moscow's invasion of Ukraine on February 24, with Russia responding in kind.
Bulgaria's foreign ministry said it had summoned Russian ambassador Eleonora Mitrofanova on Tuesday to inform her of Sofia's decision.
Bulgaria expects Russia to temporarily close its consulate general in Ruse in northern Bulgaria, while Bulgaria will temporarily close its consulate in the Russian city of Yekaterinburg, it said.
Bulgaria already expelled 10 Russian diplomats in March over the war in Ukraine.
The EU and NATO member, a once staunch ally of the Soviet Union under communism, still has close cultural, historic and economic ties with Russia.
But a series of espionage scandals since 2019 has soured relations between the two countries and seen some 20 diplomats and a technical assistant expelled.