Sanna Marin on Saturday told Social Democratic Party delegates gathered in Helsinki that it was time for Finland to seriously reconsider its stance on military allegiance.
Russia's military aggression in Ukraine has forced Finland to reexamine its security policy, according to Marin, who said, "Russia is not the neighbour we thought it was."
Any decision to seek Nato membership would have to be made "thoroughly but quickly," essentially this spring, the PM said in her speech to party top brass.
She said Finland would face consequences both if it chose to seek accession to the alliance or opted to stay out of it.
The premier said she would reveal her personal stance on Nato as discussions on membership unfold in the coming weeks.
"If the president or I voiced strong opinions on the issue, it would be the end of the debate...I believe it's very important that all of Finland's most central institutions are involved in the ongoing debate," she said.
Marin also stressed that she was unaware of any current Nato members opposing a potential bid by Finland.
SDP delegates gathered in Helsinki on Saturday to formulate the party's position on whether Finland should seek Nato membership.
Finns' interest in joining the military alliance has risen dramatically since Russia's military attack on Ukraine.