The move comes despite warnings from Russian president Vladimir Putin, who told his Finnish counterpart that joining Nato would be a "mistake".
President Sauli Niinisto called it a "historic day". He says the shift in policy is in response to Russia's invasion of Ukraine.
Nato foreign ministers say they want a rapid accession process for Finland.
Sweden could also announce its intention to join as soon as Monday.
Finland shares a 1,300-km (810-mile) border with Russia. Until now, it has stayed out of Nato to avoid antagonising its eastern neighbour.
