Joe Biden, 78, became the oldest US president in history at a scaled-back ceremony in Washington that has been largely stripped of its usual pomp and circumstance, due both to the coronavirus and security concerns following the January 6 assault on the US Capitol by Trump supporters.
“Democracy has prevailed,” he said after taking the oath of office from Chief Justice John Roberts.
Biden arrived at the Capitol just before 10:30 (1530 GMT) for his inauguration after a visit to church, where he was joined in a show of unity by the two most senior Republicans in Congress: Senator Mitch McConnell and House of Representatives Minority Leader Kevin McCarthy.
With only a small number of attendees present, he took the oath of office before US Chief Justice John Roberts just after noon (1700 GMT), placing his hand on an heirloom Bible that has been in the Biden family for more than a century.
Vice-President-elect Harris was sworn in ahead of Biden – becoming the first woman and the first black and Asian-American person elevated to serve in a role a heartbeat from the presidency.
The inauguration took place at the US Capitol with extra-tight security after the building was stormed by violent pro-Trump protesters in a deadly riot on January 6.
Some 25,000 National Guards protected the inauguration ceremony, which is missing the traditional hundreds of thousands of spectators because of the coronavirus pandemic.