The Federal Security Service said a Romanian diplomat was caught spying Monday and given 48 hours to leave the country.
Gabriel Grecu, first secretary in the political department of the Romanian Embassy in Moscow, was detained while trying to obtain secret military information from a Russian citizen, the FSB said in a statement.
"Espionage equipment seized from the agent fully reveals his hostile activities against Russia," the statement said.
Russia has made a formal protest to the Romanian government about Grecu's activities and declared him persona non grata, the FSB said.
The Romanian Embassy in Moscow and Romania's Foreign Ministry in Bucharest both declined to comment on the matter.
Relations between Russia and Romania are cool. Romania joined NATO in 2004, and it has annoyed Russia by supporting the pro-European government in Moldova, a former Soviet republic where two-thirds of the population speaks Romanian.
In June, Moldova ordered Russia to withdraw 1,500 troops who have been stationed in a separatist region of the country since the Soviet Union collapsed nearly two decades ago.