Sweden officially became the 32nd member of NATO after its government submitted the ratification documents on Thursday.
A formal notice issued by the U.S. State Department said that on Thursday, the "Protocol to the North Atlantic Treaty on the Accession of the Kingdom of Sweden" had entered into force.
Swedish accession means all NATO allies – including the United States – are committed to defending the country if it is attacked, and vice versa. Sweden also brings with it a sizable defense industry that already supplies several NATO countries.
NATO Secretary General Jens Stoltenberg hailed Sweden becoming a member of the alliance on Thursday as "historic", saying the Nordic country now had the ultimate guarantee of security.
Sweden became NATO's newest member upon depositing its instrument of accession to the North Atlantic Treaty with the Government of the United States in Washington.
"After over 200 years of non-alignment Sweden now enjoys the protection granted under Article 5, the ultimate guarantee of Allies' freedom and security," Stoltenberg said in a statement on X.
"Sweden brings with it capable armed forces and a first-class defense industry. Sweden's accession makes NATO stronger, Sweden safer and the whole Alliance more secure," he added.
Sweden and Finland both asked to join NATO in May 2022, following Russia's full-scale invasion of Ukraine in February of that year. Finland became a member of the alliance in April 2023.
The Swedish handover of accession documents will take place at a ceremony in Washington later on Thursday. Sweden will immediately become NATO's 32nd member when it deposits the formal documentation.
Swedish Prime Minister Ulf Kristersson was visiting Washington on Thursday.
Russia's February 2022 invasion prompted Sweden and neighboring Finland to apply to join the bloc, ending a long-standing stance of non-alignment in both countries.
In a post to X on Thursday just hours before the ceremony, Kristersson said NATO Secretary-General Jens Stoltenberg had informed him "that all NATO member states have accepted our accession protocol, and has invited Sweden to accede to the North Atlantic Treaty."
"Sweden will soon be NATO's 32nd member," Kristersson said.
The Swedish prime minister is expected to attend the annual State of the Union address by President Joe Biden, who has been struggling to persuade the rival Republican Party to approve new aid to Ukraine.
Sweden's blue and golden-yellow flag is expected to be hoisted on Monday at the Brussels headquarters of the North Atlantic Treaty Alliance.
Russia has vowed "countermeasures" over Sweden's entry into NATO, especially if the alliance's troops and assets deploy in the country.
Sweden and Finland, while both militarily intertwined with the United States and members of the European Union, have historically steered clear of officially joining NATO, formed in the Cold War to unite against the Soviet Union.