In his first speech, Biden says "this is democracy's day" and says the US "has much to do in this winter of peril, much to repair".
Joe Biden has been sworn in as the 46th US president at the US Capitol in Washington.
Vice-president Kamala Harris has taken her oath too – becoming the first woman to hold that position.
In his first speech, Biden says "this is democracy's day" and says the US "has much to do in this winter of peril, much to repair".
"I thank my predecessors from both parties who are here today," says the new president. Former Presidents Bill Clinton, George Bush and Barack Obama are attending the ceremony.
Biden says he spoke to President Jimmy Carter - who is 96 - by phone, and salutes him for his lifetime of service.
His immediate predecessor, Donald Trump, chose not to attend the inauguration.
Meanwhile, Republican Senator Roy Blunt referenced the January 6 attack at the US Capitol, saying that it “reminds us that a government to balance and check itself, is both fragile and resilient.”
Blunt called the ceremony a moment to unify the United States after the attack, CNN reports.
“This is not a moment of division. It's a moment of unification. A new administration begins and brings with it a new beginning. And with that, our great national debate goes forward and a determined democracy will continue to be more essential in pursuit of a more perfect union and a better future for all Americans,” he added.