The Finnish flag is due to be raised at NATO headquarters in Brussels, to mark Russia’s western neighbor becoming the 31st member of the Western alliance, the BBC reports.
Finnish President Sauli Niinisto and the US Secretary of State will join NATO ministers for a joining ceremony.
The length of Russia’s border with NATO member states has now been doubled.
Finland shares a 1,340-km (832-mile) eastern frontier with Russia and formally applied to join the Western security alliance with Sweden last May because of the war in Ukraine.
Sweden’s application has for now become stuck, with Turkish President Recep Tayyip Erdogan accusing Stockholm of embracing Kurdish militants and allowing them to demonstrate on the streets. Hungary is also yet to approve Sweden joining.
Helsinki’s road to accession has lasted under a year, and Tuesday’s ceremony coincides with the 74th anniversary of NATO’s founding in 1949.
It will formally join when Foreign Minister Pekka Haavisto hands over accession papers to US Secretary of State Antony Blinken.