A Russian military aircraft is suspected of having entered Finnish airspace over the towns of Kesälahti and Parikkala near the two countries' border, the Finnish Defence Ministry said.
The border guard is investigating what happened, the ministry said.
The aircraft was a Mi-17 military helicopter, which is said to have flown 4 to 4.5 kilometres into Finnish airspace, broadcaster Yle reported.
Finland and Russia share a border of over 1,300 kilometres and Wednesday's incident occurred amid intense debate in Finland and Sweden over whether to join NATO, a question that has gained new urgency since Russia began its invasion of Ukraine.
A Russian military aircraft briefly violated Swedish and Danish airspace a few days ago.