Nato secretary general Jens Stoltenberg and Finnish president Sauli Niinisto took part in a ceremony to mark Finland's accession to the military alliance on Tuesday, 4 April.
The Northern European country - which borders Russia - applied to join the military alliance following Vladimir Putin's invasion of Ukraine.
Pekka Haavisto, the Finnish foreign minister, has signed the accession document and given it to US Secretary of State Antony Blinken.
Finland will now be protected by Nato's Article Five, meaning that if the country was invaded or attacked it would be aided by all Nato members.
"It will be a good day for Finland’s security, for Nordic security, and for Nato as a whole,” Mr Stoltenberg said ahead of the ceremony.
Moscow has responded by promising to boost troop number in its western and northwestern regions.
Kremlin spokesperson Dmitry Peskov labelled the move as a "violation of our security and our national interests".
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