Russia announced Thursday it was expelling a Romanian diplomat in response to Bucharest doing the same to a Russian envoy last month.
Relations between Russia and the former Eastern Bloc country have deteriorated since the Ukraine war began in 2022.
In May, the NATO member accused Russia of firing a drone that hit a block of flats in the Romanian city of Galati, a charge Russia rejected.
Romania expelled a Russian diplomat in July over the drone incursions.
"Romania's Charge d'Affaires ... was handed a note from the Ministry declaring an employee of the Romanian Embassy in Moscow persona non grata," Russia's foreign ministry said.
"This measure is a response to the Romanian side's earlier, unjustified decision to declare a member of staff at the Russian Embassy in Bucharest persona non grata," it added.
The drone crash in May, which wounded two people, sparked fury from Romania and its NATO and EU allies.
Drone incursions in Romania have been detected dozens of times since the start of the Ukraine war, but no residential buildings had previously been hit.