Romania is expelling the deputy military attaché at the Russian embassy for activities incompatible with his diplomatic status, the foreign ministry said.
The ministry gave no further reason for ordering out the diplomat, Alexey Grishayev. Russia's TASS news agency quoted the Russian ambassador as saying Moscow reserved the right to respond.
Seven other former Soviet bloc countries in Central and Eastern Europe, all of them members of the European Union and the Western defence alliance NATO, have expelled Russian diplomats in recent weeks, triggering reciprocal measures by Moscow.
Most dramatically, Prague ordered out 18 Russian diplomats on April 17 after finding that Russian intelligence operatives were likely to have caused a blast at a Czech ammunition depot in 2014 that killed two people - an allegation Moscow denied.
Also on Monday, Russia expelled Italy's deputy naval attaché. Moscow said it was responding to Italy's expulsion of two Russian diplomats last month after police said they had caught an Italian navy captain passing secret documents to a Russian military official in return for money.