He took the oath of office in the rotunda of the US Capitol.
The inauguration ceremony was moved indoors for the first time in decades due to the bitter cold.
JD Vance was also sworn in as vice-president.
The inauguration coincided with the federal holiday honouring the Rev. Martin Luther King Jr.
Presidents Bill Clinton and Barack Obama were also sworn in for their second terms on Martin Luther King Jr Day.
Tech billionaires, cabinet nominees and former presidents are among those attending the ceremony.
Wife of former president Barack Obama, Michelle, chose not to attend.
And while former Vice President Mike Pence is in attendance, his wife, former second lady Karen, did not.
