Finland has become the 31st member of the Nato security alliance, doubling the length of member states' borders with Russia.
The Finnish foreign minister handed the accession document to the US secretary of state who declared Finland a member.
Then in bright sunshine in front of Nato's gleaming new headquarters, Finland's white-and-blue flag joined a circle of 30 other flags.
Finland's accession is a setback for Russia's Vladimir Putin.
He had repeatedly complained of Nato's expansion before his full-scale invasion of Ukraine.
US Secretary of State Antony Blinken said by attacking his neighbour, the Russian leader had triggered exactly what he had sought to prevent.
Kremlin spokesman Dmitry Peskov warned that Russia would be "watching closely" what happens in Finland, describing Nato's enlargement as a "violation of our security and our national interests".
A military band played Finland's national anthem followed by the Nato hymn. Beyond the perimeter fence a small group of protesters waving Ukrainian flags chanted "Ukraine in Nato", a reminder of why non-aligned Finland had asked to join along with Sweden in May 2022.
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