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”.
Nato today “is stronger than ever,” US President Joe Biden said.
“Nato stands more united, determined, and dynamic” the US leader said, adding that “together with our newest ally Sweden – Nato will continue to stand for freedom and democracy for generations to come.”
“This has been a little bit of a road but we’ve known from day one that we would be here one day,” US Secretary of State Antony Blinken said.
Nato Secretary-General Jens Stoltenberg posted 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”.
Ukraine’s President Volodymyr Zelensky also welcomed Sweden’s accession to Nato, saying “one more country in Europe has become more protected from Russian evil.”
Russia has vowed to take unspecified political and military measures in response to Sweden’s move.
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.
Turkey initially withheld approval in a row over what it called Sweden’s support to Kurdish separatists. It eventually lifted its veto in January of this year.
Hungary’s Prime Minister Viktor Orban accused Sweden of being hostile to it and delayed its approval until last week, when the Hungarian parliament eventually voted to ratify the bid.
All Nato members are expected to help an ally which comes under attack.
Mr Blinken recalled Sweden’s 200-year policy of non-alignment which lasted until Russia invaded Ukraine, saying that Nato expansion was not “foreordained” or “foreseeable”.
Mr Kristersson said: “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 added that security situation in the region had not been this serious since World War Two, and added that Sweden was joining Nato both to provide and to gain security.
Finland formally joined in April last year, doubling the length of the alliance’s border with Russia.
On Monday a flag-raising ceremony to mark Sweden’s accession will take place at the Nato headquarters in Brussels.