The military alliance will welcome Finland as its 31st member in a flag-raising ceremony at NATO headquarters on the outskirts of Brussels, attended by Finnish President Sauli Niinisto and government ministers.
"It will be a good day for Finland's security, for Nordic security and for NATO as a whole," NATO Secretary-General Jens Stoltenberg told reporters on Monday.
The event marks the end of an era of military non-alignment for Finland that began after the country repelled an invasion attempt by the Soviet Union during World War Two and opted to try to maintain friendly relations with neighboring Russia.
But Russia's recent invasion of another neighbor, Ukraine, which began in February 2022, prompted Finns to seek security under the umbrella of NATO's collective defense pact, which states that an attack on one member is an attack on all.
Sweden underwent a similar transformation in defense thinking and Stockholm and Helsinki applied together last year to join NATO.
But Sweden's application has been held up by NATO members Turkey and Hungary.
After both those countries approved Finland's application last week, the final formal step on Helsinki's journey will come when Foreign Minister Pekka Haavisto hands his nation's accession document to US government officials in Brussels.
Finland's flag will then be raised outside NATO headquarters alongside those of the alliance's 30 other member countries before a gathering of NATO foreign ministers.
Finland's accession roughly doubles the length of the border that NATO shares with Russia. Moscow said on Monday it would strengthen its military capacity in its western and northwestern regions in response to Finland joining NATO.
Even before Finland formally joined the alliance, its armed forces have been drawing closer to NATO and its members.
NATO's surveillance flights by the US and other allied air forces have already began to circulate in Finnish airspace, the Finnish defense forces said.
On March 24, air force commanders from Sweden, Norway, Finland and Denmark said they had signed a letter of intent to create a unified Nordic air defense aimed at countering the rising threat from Russia.
"We would like to see if we can integrate our airspace surveillance more, so we can use radar data from each other's surveillance systems and use them collectively," Major General Jan Dam, commander of the Danish air force, told Reuters.
Finns enjoying spring sunshine in downtown Helsinki on Monday said they were pleased the NATO membership process would soon be complete, even if some harbored reservations.
"I feel maybe a little conflicted about joining NATO because I'm not the biggest fan of NATO but at the same time even less a fan of Russia," said Henri Laukkanen, a 28-year-old financial assistant.
Finland and Sweden had said they wanted to join NATO "hand in hand" to maximize their mutual security but that plan fell apart as Turkey refused to move ahead with Stockholm's bid.
Turkey says 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.
Hungary is also holding up Sweden's admission, citing grievances over criticism of Prime Minister Viktor Orban's democratic record.
But NATO diplomats say they expect Budapest to approve Sweden's bid if it sees Turkey moving to do so. They hope Turkey will move after presidential and parliamentary elections in May.
Stoltenberg said he was "absolutely confident" that Sweden will become a NATO member.
"It's a priority for NATO, for me, to ensure that happens as soon as possible," he said.
