The Finnish flag is due to be raised at Nato headquarters in Brussels, to mark Russia's western neighbour becoming 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 ahead of his full-scale invasion of Ukraine.
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 Russia's war.
They had previously both adopted a policy of non-alignment. But in the face of an increasingly belligerent Russian military, they preferred to move to the protective guarantee provided by Nato's Article Five, which says an attack on one member is an attack on all.
Russia's invasion prompted a surge in Finnish public opinion towards joining Nato to 80% in favour.
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.
"This is really an historic day. It's a great day for the alliance," Nato Secretary General Jens Stoltenberg said ahead of the ceremony.
"Finland's a terrific ally, very capable, shares our values and we expect a seamless transition into its proper seat at the table," US ambassador to Nato Julianne Smith told the BBC. She said she hoped Sweden would also join at the next Nato summit in Lithuania in July.
Russia's Deputy Foreign Minister, Alexander Grushko, has warned that if Finland's new Nato allies deploy forces or resources there, Moscow "will take additional steps to reliably ensure Russia's military security".
On Sunday, Russia's ambassador to Belarus, Boris Gryzlov, said Moscow would move tactical nuclear weapons close to Belarus's western borders to "increase the possibilities to ensure security". However, Nato's secretary general said it had not yet seen any changes to Russia's nuclear posture.
Nato will now have seven members on the Baltic Sea, further isolating Russia's coastal access to St Petersburg and its small exclave of Kaliningrad.
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