The son of the Philippines' late dictator Ferdinand Marcos succeeds the hugely popular Rodrigo Duterte, who gained international infamy for his deadly drug war and has threatened to kill suspected dealers after he leaves office.
Popularly known as "Bongbong", Marcos Jr, 64, won last month's elections by a landslide, securing the biggest victory since his father was ousted by a popular revolt in 1986.
Marcos Jr took the oath at midday in a public ceremony at the National Museum in Manila in front of hundreds of local and foreign dignitaries, including Chinese Vice President Wang Qishan and US Second Gentleman Douglas Emhoff.
"This is a historic moment for us all," said Marcos Jr after being sworn into office. "By your vote, you rejected the politics of division."
With his 92-year-old mother Imelda sitting metres away, Marcos Jr also praised his late father's regime, which critics describe as a dark period of human rights abuses and corruption.
"I once knew a man who saw how little had been achieved since independence ... He got it done," Marcos Jr said, claiming his father built more roads and produced more rice than all of his predecessors combined.
"So will it be with his son. You will get no excuses from me."
More than 15,000 police, soldiers and coast guard personnel were deployed across the capital for the inauguration.
Ahead of the swearing-in, Duterte received Marcos Jr at the Malacanang presidential palace - which the Marcos family fled into exile 36 years ago.
Duterte, 77, wore a mask and his traditional formal shirt, characteristically unbuttoned at the top and with sleeves rolled up, for the meeting with Marcos Jr.
The ceremony comes days after the Supreme Court dismissed final attempts to have Marcos Jr disqualified from the election and prevent him taking office.