Ambassadors from NATO's 30 member states have signed accession protocols for Sweden and Finland.
The move opens the months-long period for the alliance to ratify their membership.
The signing at the NATO headquarters follows the deal with Turkey last week in Madrid, where Ankara lifted its veto on the Nordic membership bids.
In the wake of the Russian-Ukraine conflict which began in February,
Sweden and Finland announced their intention to drop their military non-aligned status and become parts of a NATO.
"This is an historic day, for Finland, for Sweden, for NATO, and for Euro-Atlantic security.
I commend all Allies for moving so quickly in accepting Finland and Sweden's applications for membership.
And I want to thank Türkiye, Finland, and Sweden for their constructive approach.
The trilateral agreement they signed at the Madrid Summit made today possible.
I know all three parties are working hard on the implementation, especially in the fight against terrorism."