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 pomp-laden 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 in the White House's Blue Room, surrounded by portraits of former presidents.
At the end of the brief and smooth 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, some 800,000 people are flocking to Washington for the event. The star-studded inauguration includes acts by Beyonce and Katy Perry.
Thousands of workers and volunteers are making final preparations for the ceremonies. Hotels and government buildings along the parade route were adorned with red, white and blue bunting.
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.
Mr Obama and the first lady yesterday participated in a day of service, hoping to encourage Americans to follow suit with volunteering projects.
Mr Obama added the day of service projects to the inaugural schedule in 2009 and he is hoping the event becomes a tradition for future presidents.
The White House says the call to service is a way for Americans to honour the memory of Martin Luther King Jr. This year marks the 50th anniversary of the civil rights leader's march on Washington.