Finnish Foreign Minister Pekka Haavisto completed the accession process by handing over an official document to US Secretary of State Antony Blinken at Nato headquarters in Brussels.
At a flag-raising ceremony outside the alliance's headquarters, Nato Secretary-General Jens Stoltenberg praised the Finnish armed forces, calling him "substantial and highly capable."
"For many years, troops from Finland and Nato countries have worked side-by-side as partners. From today, we stand together as allies," said the former Norwegian prime minister.
Finnish President Sauli Niinisto called for a quick accession of its neighbour Sweden, which membership bid has been held off by Ankara, which says Stockholm has not gone far enough in cracking down on people Turkey considers terrorists.
Turkey has repeatedly said that Sweden needed to take additional steps against supporters of Kurdish militants and members of the network Ankara holds responsible for a 2016 coup attempt. Turkey treats both groups as terrorist organisations.
Finland's membership represents the first enlargement since North Macedonia joined the alliance in 2020.
Finnish foreign minister takes part in first Nato meeting as a member
Shortly after the ceremony, Finnish Foreign Minister Haavisto joined his Nato counterpart for a first meeting as a representative of a Nato member at the beginning of which he received an ovation.
"You have attended many meetings, but this is the first time you sit down there between France and Estonia and it's a great privilege to have you now as a full-fledged (Nato) member, Nato Secretary-General Jens Stoltenberg told him.
The former Norwegian prime minister said Sweden would be joining the alliance soon.
The United States and other Nato countries are hoping that Sweden becomes a member of the alliance at a Nato summit due to be held on July 11 in Lithuania's capital Vilnius.
The country's accession brings Nato significant military capabilities developed over the years as it is one of the 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 attack.
Turkey welcomes Finland to Nato as talks over Sweden's membership continue
Turkish Foreign Minister Mevlut Cavusoglu handed Finland's Nato ratification document to US Secretary of State Antony Blinken on Tuesday on the sidelines of the Brussels summit.
Turkey's parliament approved the bill for Finland's accession to Nato last week, clearing the way for the country to become part of the Western defence alliance as war rages in Ukraine.
The Turkish parliament was the last among the 30 members of the alliance to ratify Finland's membership after Hungary's legislature approved a similar bill earlier this week.
Cavusoglu also met with his Finnish and Swedish counterparts to discuss bilateral relations.
President Tayyip Erdogan said earlier in March that Finland had secured Turkey's blessing after taking concrete steps to keep promises to crack down on groups seen by Ankara as terrorists, and to free up defence exports.
Finland and Sweden asked to join Nato last year in response to Russia's invasion of Ukraine. But the process has been held up by Turkey and Hungary. The parliaments of all Nato members must ratify newcomers.
While Finns head into the alliance, close partner Sweden continues to wait for ratification of its Nato membership bid in the face of opposition from Turkey and Hungary.
Finns speak of a 'special day' as the Nato flag flies for the first time
People across Finland took stock of the reality of Nato membership on Tuesday with a mix of acclaim and equanimity as the geopolitical fallout played out from Moscow's invasion of a different neighbour, Ukraine.
Retired Finnish combat engineer Ilkka Lansivaara, sporting camouflage cap and fatigue pants, had hung his own Nato flag from the side of the squat brick house where he lives in Virolahti, near the Russian border due east of Helsinki.
"It's a special day for Finland of course," said Lansivaara, 70, who served in Nato-led forces in Bosnia in the late 1990s and whose father was an air force pilot during World War Two.
"This morning when I woke, first I took a mug of coffee, drank it and then I lifted the flag and then another mug of coffee and then I took some pictures of the flag."
In nearby Vaalimaa, once a busy crossing point for Finns and Russians alike, located roughly midway between Helsinki and St Petersburg, border traffic was a slow trickle. Few expect it to bounce back any time soon.
It was a far cry from the scenes last year when thousands of Russians fled conscription before the Finns closed the border to tourists in September.
Nearby shops are quiet and a mall, popular with Russians, has closed.
Down the road, Matti Seppala, 78, a retired warehouse worker who grew up near the border said joining would bolster Finland's security even if he was not scared of Russian "sabre rattling".
"I supported joining, you never know with the big brother, what they will do," he said.
The Ukraine invasion, which Moscow calls a "special military operation, has led both Finland and Sweden to abandon decades of military non-alignment and seek safety in the Nato camp.
The accession of Finland with its 1,300-km (810-mile) border with Russia roughly doubles Nato's land frontier facing Moscow at a time when East-West relations have hit their lowest point in decades.
Finland brings a sizable, well-trained military into Nato and Russia, though bogged down in gruelling fighting in Ukraine, has said it will have to take "counter-measures" to ensure Russian security in response.
Reuters