"The golden age of America begins right now. From this day forward our country will flourish and be respected again all over the world," Trump said in the U.S. Capitol, where the ceremony was held inside for the first time in decades due to frigid weather.
While promising renewal, Trump's tone was characteristically dark, denouncing what he said had been a "betrayal" of Americans by a "radical and corrupt establishment".
"For many years, a radical and corrupt establishment has extracted power and wealth from our citizens, while the pillars of our society lay broken and seemingly in complete disrepair," he said.
"From this moment on, America's decline is over."
The Republican - at 78 now the oldest person ever to take the presidential oath - was set to kickstart his new term with ablitz of immediate orders on immigrationand the U.S. culture wars.
"I will declare a national emergency at our southern border" with Mexico, Trump said to loud cheers from supporters inside the ornate Rotunda hall, vowing to deport "millions and millions" of illegal immigrants.
Trump took the oath with one hand raised in the air, using a Bible given to him by his mother, and became the 47th president.
He had travelled to the Capitol with outgoing Democratic president Joe Biden, who followed tradition by offering his successor tea at the White House.
"Welcome home," Biden had said to Trump as he and First Lady Jill Biden greeted the incoming president and his wife Melania at the White House.
Trump was a political outsider at his first inauguration in 2017 as the 45th president, but this time around he is surrounded by America's wealthy and powerful.
The world's richest man, Elon Musk, Meta boss Mark Zuckerberg, Amazon chief Jeff Bezos and Google CEO Sundar Pichai all had prime seats in the Capitol alongside Trump's family and cabinet members.
Musk, who bankrolled Trump's election campaign to the tune of a quarter of a billion dollars and promotes far-right policies on the X social network, will lead a cost-cutting drive in the new administration.
While Trump refused to attend Biden's 2021 inauguration after falsely claiming electoral fraud, this time Biden has been keen to restore the sense of tradition.
Biden joined former presidents Barack Obama, George W Bush and Bill Clinton at the Capitol. Former first ladies Hillary Clinton and Laura Bush were there but ex-first lady Michelle Obama pointedly stayed away.