Sweden has officially joined Nato after completing its accession process, ending decades of post-Second World War neutrality.
The handover of documents took place at a ceremony two years after Sweden applied to join the military alliance following Russia's full-scale invasion of Ukraine.
Swedish Prime Minister Ulf Kristersson said that "unity and solidarity" would be Sweden's "guiding lights".
“We are humble, but we are also proud. We will live up to all expectations."
"We share burdens, responsibilities and risk with other allies,” he said.
US Secretary of State Antony Blinken said: “Let me welcome Sweden” as the 32nd member of the military alliance.
He said that the Russian invasion of ukraine had triggered the move.
Blinken added: “Sweden had a 200 year-old policy of non alliance ... after the invasion (of Ukraine) three quarters of Swedish people said they wanted to join.”
"Good things come to those who wait," US Secretary of State Blinken also said.
"Our defensive alliance is now stronger and larger than it's ever been," he added, thanking Sweden for its "resilience".
"This has been a little bit of a road but we've known from day one that we would be here one day," Mr Blinken said.
Nato Secretary-General Jens Stoltenberg said in a statement on X that Sweden brings with it "capable armed forces and a first-class defence industry" and that the alliance had become "stronger and safer".
Sweden applied to join the defence alliance after Russia's full-scale invasion of Ukraine in 2022 - but its request was blocked by two members. Hungary Prime Minister Viktor Orban had initially attempted to block the move.
Turkey also opposed the move over what it perceived as Sweden's support to Kurdish separatists. It eventually lifted its veto in January of this year.
Finland formally joined in April last year in the face of Russian aggression.
All Nato members are expected to help an ally which comes under attack.
