The Bulgarian Foreign Ministry said on Thursday in a statement that it has declared a diplomat at the Russian Embassy in Sofia persona non grata, Trend reports citing Xinhua.
The statement did not mention the grounds for the expulsion.
The Russian Embassy in Sofia confirmed the expulsion, saying in a statement that an assistant military attache was declared persona non grata.
"The Russian side reserves the right to retaliate," it added.
On March 22, Bulgaria expelled two Russian diplomats for alleged espionage.