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.
Russia’s invasion prompted a surge in Finnish public opinion towards joining Nato to 80% in favour.
“This will make Finland safer and Nato stronger,” Nato Secretary General Jens Stoltenberg told reporters on Wednesday.
“President Putin had a declared goal of the invasion of Nato to get less Nato along its borders and no more membership in Europe, he’s getting exactly the opposite.”
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.
Mr Stoltenberg said Nato would ensure Sweden would become the next member to join.
Helsinki’s journey 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.
“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”.
Defence Minister Sergei Shoigu said on Wednesday that Russia’s short-range Iskander-M ballistic missile system had been handed over to Belarus and was capable of carrying nuclear as well as conventional weapons.
However, Mr Stoltenberg said Nato had not yet seen any changes to Russia’s nuclear posture that would require any change by the alliance. He added there would be no Nato troops stationed in Finland without the consent of the government in Helsinki.
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.