Crown Prince Maha Vajiralongkorn, 64, became Thailand's new king late on Thursday after he accepted an invitation from parliament to succeed his father, King Bhumibol Adulyadej, who died in mid-October.
The new king inherits one of the world's richest monarchies, as well as a politically febrile nation, 50 days after King Bhumibol’s death. His father was widely loved and regarded as a pillar of stability during decades of political turbulence and rapid development in the Southeast Asian nation.
"I agree to accept the wishes of the late king... for the benefit of the entire Thai people," said Vajiralongkorn, wearing an official white tunic decorated with medals and a pink sash, in a televised statement from Bangkok.
Thailand has been without a monarch since King Bhumibol's death, with the head of the king's advisory council, Prem Tinsulanonda, 96, serving as regent.
The United States, a longtime ally of Thailand, sent its "warmest congratulations" to Vajiralongkorn.
"We offer our best wishes to his majesty and all of the Thai people," State Department spokesman Mark Toner said at a news briefing.
Makes first public appearance
On Friday, the new king made his first public appearance, taking part in a merit-making ceremony at Bangkok's Grand Palace to mark 50 days since his father's death plunged the country into grief.
Civil servants dressed in black and white, the official colours of mourning, lined the streets to the palace as the new king's convoy passed.
The new king has yet to command the kind of adoration that his father received from Thais. He has kept a much lower profile throughout most of his adult life and has a home in Germany.
Despite the popularity the late king Bhumibol had enjoyed, his reign marked a tumultuous period of Thai history pockmarked by a communist insurgency, coups and street protests
The military toppled an elected government in 2014 and enforced political calm in a country divided by more than a decade of conflict between a military-backed royalist establishment and populist political forces.
In times of crisis, the palace has occasionally acted as a final arbiter. The late king's image as a truce-maker peaked after violent clashes in 1992 between pro-democracy activists and the army.
"Some people have pointed to the monarchy as a stabilising force in Thai politics," said David Streckfuss, an independent academic specialising in Southeast Asian studies.
"Because of the popularity and veneration of the previous king it would be hard for any person to step into the role because of expected comparisons," he said.
The new king, like his father before him, is shielded from public criticism by Thailand's lese majeste laws, which carry a penalty of up to 15 years in prison.
Thailand's constitutional monarchy has limited formal powers but it draws the loyalty of much of the kingdom's business elite, as well as a military that dominates politics through its regular coups.