The Finnish flag is due to be raised at Nato headquarters in Brussels, as Russia's western neighbour becomes the 31st member of the Western alliance.
Finnish President Sauli Niinisto and the US Secretary of State will join Nato ministers for a joining ceremony.
Finland's accession is a setback for Russia's Vladimir Putin, who repeatedly complained of Nato's expansion before his full-scale invasion of Ukraine.
The length of Russia's border with Nato member states has now doubled.
Finland shares a 1,340-km (832-mile) eastern frontier with Russia and formally applied to join Nato with Sweden last May because of Russia's war.
They had previously both adopted a policy of non-alignment. But after the Ukraine invasion, they chose the protection of Nato's Article Five, which says an attack on one member is an attack on all.
In effect, it means if Finland were invaded, all Nato members - including the US - would come to its aid.