Joe Biden has been sworn in as the 46th US president, ending one of the most dramatic political transitions in American history.
“Democracy has prevailed,” he said after taking the oath of office from Chief Justice John Roberts.
Donald Trump, who has not formally conceded to Mr Biden, did not attend the ceremony.
Kamala Harris was sworn in as vice-president ahead of Mr Biden.
She is the first woman – and the first black and Asian-American person – to serve in a role, a heartbeat from the presidency.
The inauguration took place at the US Capitol. There is extra-tight security after the building was stormed by violent pro-Trump protesters in a deadly riot on 6 January.
In his inaugural address, Mr Biden said it was a day of “history and hope”.
“My whole soul is in putting America back together again,” he added.
Highlighting a message of unity after the turbulent Trump years, he promised to be a president “for all Americans” – including those who voted against him.
Among those who attended the ceremony were three of his predecessors: Barack Obama – under whom Mr Biden served for eight years as vice-president – Bill Clinton and George W Bush.
Former Vice-President Mike Pence also attended. He skipped Mr Trump’s farewell military salute event.
The inauguration ceremony included musical performances by Lady Gaga – who sang the national anthem – as well as Jennifer Lopez and Garth Brooks.
Earlier on Wednesday Mr Biden, 78, attended Mass at a cathedral in Washington – along with four Roman Catholic congressional leaders, both Democrats and Republicans.
Trump was the first president not to attend his successor’s inauguration since 1869.
He left the White House at about 08:00 (13:00 GMT) on Wednesday, and flew to the nearby Andrews Air Force base.
The 74-year-old then left for his Mar-a-Lago golf club in Florida, where he arrived later in the morning.