What does Finland's membership of Nato mean for Russia? ITV News Security Editor Rohit Kachroo reports
Previously the country had not joined any military alliances, but in 2022 it sought protection from neighbouring Russia's aggression.
Finnish Foreign Minister Pekka Haavisto handed over the document, which officially enshrined the decision, to US Secretary of State Antony Blinken on Tuesday. A flag-raising ceremony is due to take place later.
British Prime Minister Rishi Sunak called it an "historic day for Finland and for Nato", saying Finnish membership had made the alliance stronger.
Earlier Moscow threatened "retaliation", warning it would bolster defences along its border with Nato if the alliance deploys any additional troops or equipment to its new member, to address what it called security threats created by Finland's membership.
The head of the military alliance said no more troops would be sent to the Nordic country unless it asked for help.
But Nato Secretary-General Jens Stoltenberg refused to rule out the possibility of holding more military exercises there, adding that Nato would not allow Russia's demands to dictate the organisation's decisions.
Speaking at the alliance's headquarters in Brussels, a few hours before Finland officially joined, he said: "We are constantly assessing our posture, our presence. We have more exercises, we have more presence, also in the Nordic area."
UK Foreign Secretary James Cleverly, who was at Nato's headquarters, said: "Today we see, as a direct result of Vladimir Putin's aggression and his illegal invasion of Ukraine, the day where a new ally joins our defensive alliance."
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He added: "Russia thought its aggression would divide us. Instead, we are bound tighter together, resolute in our defence of the principles of freedom and the rule of law.
"Let us be clear that our door remains open. We will welcome further Allies with open arms and we continue to push for Sweden's swift accession."
Hours before Finland officially entered the alliance, it's parliamentary website was hacked, in an attack which was claimed by a pro-Russian hacker group, known as NoName057 (16).
Finland has officially become the 31st member of Nato and take its place among the ranks of the world's biggest security alliance.
Alarmed by Russia's invasion of Ukraine last year, Finland applied to join in May, setting aside years of military non-alignment to seek protection under the security umbrella.
Neighbouring Sweden also applied, but its accession process may take a few months longer.
Finland shares a 832 mile border with Russia, so its entry will more than double the size of Nato's border with the country.
The move is a strategic and political blow to Russian President Vladimir Putin, who has long complained about Nato's expansion toward Russia - though the alliance says it poses no threat to Moscow.
"The Russian Federation will be forced to take military-technical and other retaliatory measures to counter the threats to our national security arising from Finland's accession to Nato," the foreign ministry warned in a statement.
It said Finland's move marks "a fundamental change in the situation in Northern Europe, which had previously been one of the most stable regions in the world".
Finland's entry falls on the organisation's 74th anniversary of the signing of its founding Washington Treaty on April 4, 1949.
It also coincides with a meeting of the alliance's foreign ministers, which Finland's president, foreign and defence ministers took part in.
Turkey became the last Nato member country to ratify Finland's membership protocol on Thursday.
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