Signing the ratification protocols on Tuesday, Biden said that NATO "is more united than ever, and after Finland and Sweden join we will be stronger than ever.”
Biden also said on Tuesday that "Finland and Sweden's decision to join NATO is a watershed moment" for the alliance.
"It will help ensure greater security and stability for our world," he added.
Last week, the US Senate voted 95 to 1 to approve the entry of Finland and Sweden into the Western military alliance.
Biden said in a statement at the time that the vote sent "an important signal of the sustained, bipartisan U.S. commitment to NATO, and to ensuring our Alliance is prepared to meet the challenges of today and tomorrow."
He hailed Finland and Sweden as "two strong democracies with highly capable militaries, into the greatest defensive alliance in history."
Poland's President Andrzej Duda last month signed legislation to ratify the expansion of NATO to include Finland and Sweden in the wake of Russia’s invasion of Ukraine.
"Today is a very important day for NATO, for our part of Europe, for the Baltic Sea region and for the security of this part of the world," Duda said at the time during a high-profile ceremony in the Baltic port city of Gdynia.
He added that the accession of Finland and Sweden to NATO was also important "for future generations" of Europeans.
The Polish president has previously said that NATO’s invitation for Finland and Sweden to join the alliance was “a historic decision.”
NATO's 30 allies on July 5 signed accession protocols for Finland and Sweden to join the Western military alliance, a move that still needs to be ratified by member states.
Polish Deputy Prime Minister and Defence Minister Mariusz Błaszczak said last month that the accession of Finland and Sweden would “strengthen NATO significantly.”
In an interview with public broadcaster Polish Radio, Błaszczak added: “It’s very advantageous for Poland’s security because it alters the situation in the Baltic Sea region.”
Polish Prime Minister Mateusz Morawiecki declared in May that Poland would come to the aid of Finland and Sweden if the two countries were attacked by Russia before formally securing NATO membership.
Twenty-three NATO members have already ratified Finland and Sweden's NATO entry, Polish state news agency PAP reported on Wednesday.
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Source: IAR, PAP, cnbc.com