One diplomat is accused of leaking "confidential information" about Bulgaria's defence ministry since 2016.
Bulgaria says it has expelled two Russian diplomats for spying amid tensions over Moscow's invasion of Ukraine.
The Bulgarian foreign ministry announced that it had received evidence that the two suspects were involved in "intelligence activities".
Ministry spokesperson Mitko Dimitrov told reporters on Wednesday that the diplomats had 48 hours to leave Bulgaria.
According to the prosecutor's office, one of the Russian diplomats had received confidential information from a Bulgarian defence ministry official since 2016.
Bulgaria has traditionally had close economic relations with Russia but tensions have heightened between the two countries since October 2019.
At least 8 Russian diplomats and one technical assistant at the embassy in Sofia have been expelled on espionage charges in the last two years.
Six Bulgarian nationals -- including military intelligence officers -- were also detained on suspicion of spying for Russia last November.
Russia is likely to respond directly to the latest expulsions, according to state media.