Without naming the diplomat, the foreign ministry said that it had communicated to the Russian Embassy in Bucharest that its first secretary must leave Romania in 48 hours.
The tit for tat statement came a day after Russia's Federal Security Service said it had detained a Romanian diplomat suspected of spying and seized espionage equipment from him.
Romania's foreign ministry condemned the detention of Gabriel Grecu, first secretary in the political department.
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 that have been stationed in a separatist region of the country since the Soviet Union collapsed nearly two decades ago.
Russia has also criticized U.S. plans to deploy missile interceptors in Romania.