Chief Justice John Roberts administered the oath of office yesterday in the Blue Room of the White House. First Lady Michelle Obama, their daughters Sasha and Malia, and a few reporters witnessed the ceremony.
Mr Obama will take the oath again today during a public swearing-in ceremony at the Capitol in front of hundreds of thousands of people.
Mr Obama was sworn in to meet a legal requirement that presidents officially take office on January 20.
He made no special remarks at the short ceremony, surrounded by portraits of former White House residents, but was heard to say: "I did it," as he embraced his wife and children.
Today, he will repeat the oath and give his inaugural speech. He will then make the traditional journey, part of it on foot, down Pennsylvania Avenue to the White House.
While Mr Obama convincingly won a second term, the jubilation that surrounded him four years ago is subdued this time around.
He guided the country through many challenges after taking office in 2009: ending the Iraq war, putting the Afghan war on a course towards US withdrawal, and saving the collapsing economy. He also won approval for a sweeping healthcare overhaul. Yet onerous problems remain, and his success in resolving them will define his place in history. He faces bitter confrontation with Republicans over gun control, avoiding a default on debts, cutting the spiralling federal deficit and stopping Iran building a nuclear weapon.
Joe Biden was also sworn in for his second term as vice-president earlier yesterday, taking the oath at his official residence at the US Naval Observatory.