Romania sent up two F16 planes to monitor the event, which occurred on Saturday in the Tulcea region in eastern Romania.
"The drone did not fly over inhabited areas and posed no immediate danger to the population," it says in a statement from the country's defense department.
The incident comes just a few days after several Russian drones violated Polish airspace on Wednesday night.
Ukraine's President Volodymyr Zelenskyj writes on X that the drone was "about 10 kilometers into Romanian territory" and that it stayed "in NATO airspace for about 50 minutes".
"An obvious expansion of the war by Russia," he continues.
Foreign Minister Maria Malmer Stenergard writes on X that "Sweden is completely in solidarity with Romania as a NATO ally and EU member state. We are always ready to contribute further to the alliance's deterrence and defense".
Romania is a member of both the EU and NATO and has a nearly 65-mile long border with Ukraine, including both land and water. Romanian airspace has been violated before by Russian drones during Russia's war of aggression against Ukraine.
