Finland's accession roughly doubles the length of the border that NATO shares with Russia and bolsters its eastern flank as the war in Ukraine grinds on with no resolution in sight.
Finnish Foreign Minister Pekka Haavisto completed the accession process on Tuesday by handing over an official document to US Secretary of State Antony Blinken at NATO headquarters in Brussels where the Finnish flag was unfurled.
"We welcome Finland to the alliance," NATO Secretary General Jens Stoltenberg said at the end of the brief ceremony.
Stoltenberg earlier noted that Russian President Vladimir Putin had cited opposing NATO's eastward enlargement as one justification for his invasion.
"He is getting exactly the opposite...Finland today, and soon also Sweden will become a full fledged member of the alliance," Stoltenberg said in Brussels.
Finnish President Saul Niinisto said Finland's most significant contribution to NATO's common deterrence and defence would be to defend its own territory. There is still significant work to be done to coordinate this with NATO, he said.
"It is a great day for Finland and I want to say that it is an important day for NATO," Niinisto said at a joint news conference with Stoltenberg.
The Kremlin said Russia would be forced to take "counter-measures" to Finland's accession. Defence Minister Sergei Shoigu said the move raised the prospect of the conflict in Ukraine escalating further.
Russia had said on Monday it would strengthen its military capacity in its western and northwestern regions in response to Finland joining NATO.
The Ukrainian government also hailed Finland's move. President Volodymyr Zelenskiy's chief of staff Andriy Yermak wrote on Telegram: "FI made the right choice. NATO is also a key goal for Ukraine."
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 neighbouring Russia.
But the invasion of Ukraine in February 2022 prompted Finns to seek security under NATO's collective defence pact, which states that an attack on one member is an attack on all.
Moscow, which long criticised the move, reacted crossly.
Kremlin spokesman Dmitry Peskov said the NATO expansion was an "encroachment on our security and on Russia's national interests". Moscow would watch closely for any NATO military deployments in Finland, he said.
Since the end of the Cold War three decades ago, Moscow has watched successive waves of NATO enlargement to the formerly communist east of Europe with consternation, and the issue was a bone of contention even before the invasion of Ukraine.
NATO has repeatedly stressed that it is solely a defensive alliance and does not threaten Russia.
Finland's accession brings NATO significant military capabilities developed over the years as it is one of few European countries to have retained a conscription army through decades of peace, wary of Russia next door. In addition, Finland's ground, naval and air forces are all trained and equipped with one primary aim - to repel any Russian invasion.
Finland and its Nordic neighbour Sweden applied together last year to join NATO, but the Swedish application has been held up by NATO members Turkey and Hungary.
Swedish Foreign Minister Tobias Billstroem told reporters it was Stockholm's ambition to become a member at the NATO summit in Vilnius in July.
"This is a question of utmost importance for Sweden...There is no reason for neither the Turkish parliament or the Hungarian parliament to make any further delays," he said.
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