Finland has a 1,300-km (810-mile) border with Russia, meaning NATO's frontier with Russia will roughly double in length, and the move drew a pledge from Moscow that it will beef up its forces in border regions.
"Tomorrow we will welcome Finland as the 31st member of NATO making Finland safer and our alliance stronger," NATO Secretary-General Jens Stoltenberg told reporters in Brussels, hailing the move as "historic".
Russia will strengthen its military capacity in its western and northwestern regions in response to Finland's expected accession to NATO, Russian Deputy Foreign Minister Alexander Grushko said, state-owned news agency RIA reported.
Finland and Sweden applied to join NATO last year after the conflict broke out in Ukraine in February last year. Finland has a 1,300-km (810-mile) border with Russia that will roughly double the transatlantic alliance's frontier facing Moscow.
Finnish President Sauli Niinisto will travel to Brussels to take part in the ceremony, his office said.
The last hurdle to Finland's membership was removed last week when Turkey's parliament voted to ratify Helsinki's application even as it kept Sweden's bid on hold.
Turkey continues to hold up Sweden's application. It says that Stockholm harbors members of what Ankara considers terrorist groups - a charge Sweden denies - and has demanded their extradition as a step toward ratifying Swedish membership.