Barack Obama is due to take a public oath of office and make his second inaugural address on the steps of the US Capitol in Washington in front of a crowd of hundreds of thousands.
Millions more across the world will tune in to see the star-studded ceremony, followed by a traditional parade and formal balls where the presidential couple will dance for the cameras.
On Sunday the US President was quietly sworn in at a small ceremony at the White House to mark the start of his second term.
Mr Obama took the oath on a family bible held by first lady Michelle Obama in the Blue Room, surrounded by portraits of former presidents.
At the end of the brief ceremony officiated by Supreme Court Chief Justice John Roberts, Mr Obama hugged his family, telling one of his daughters: "I did it".
There was no repeat of 2009, when he stumbled over the oath, prompting the White House to request a redo the following day.
The mood on the streets of the capital is less festive than it was in January 2009, when the swearing-in of the country's first black president drew a record 1.8 million people.
High unemployment, a bruising election campaign and partisan fights over fiscal policies have taken a toll.
Still, between 500,000 and 700,000 people are expected to flock to Washington for the event. The inauguration includes acts by Beyonce and Katy Perry.
Thousands of workers and volunteers have been making final preparations for the ceremonies. Hotels and government buildings along the parade route were adorned with red, white and blue bunting.
The President and his family began inauguration day by attending services at St John's Episcopal Church near the White House.
A quirk in the calendar this year pushed the public swearing-in onto the national holiday honouring the slain civil rights leader Martin Luther King Jr.
In his inaugural address, Mr Obama is expected to talk about the need for political compromise where possible, as well as detailing the priorities and goals of his second term.
The president's legacy will be tested by foreign crises, including the war in Syria and Iran's nuclear programme, as well as domestic issues, such as immigration and a weak economy.
Mr Obama is also seeking new restrictions on guns and ammunition.
He will face strong opposition by the Republican Party, as the country remains highly polarised.
Away from the politics, the President has given his opinion on what he jokingly called the most significant event of the inaugural weekend: his wife's new haircut.
"I love her bangs," Mr Obama said. "She looks good. She always looks good."
First lady Michelle Obama unveiled the new fringe last week to coincide with her 49th birthday.
And it has been the talk of the town and social media ever since.