Finland said it suspects a Russian military plane crossed into its airspace June 26, the latest in a string of reported violations in the Nordic region.
Countries on Russia's north-western flank, including the Baltic states, have reported several Russian aircraft and ships near their territory in recent months, raising fears that Moscow is trying to reassert its Cold War dominance there.
The plane was spotted south of the Finnish city of Porvoo and identified as a Russian Air Force IL-76 transport plane, Finland's defence ministry said.
The Finnish Border Guard would formally investigate the incident, it added. Such investigations typically take several weeks.
There was no immediate comment from Russia, though it denied reports of a similar violation of Finnish airspace in August.
Last year, Finland claimed five violations of its air space by Russian warplanes.