The prince accepted the throne in a TV broadcast following an invitation from parliament, formalizing his accession.
King Bhumibol, the world’s longest-reigning monarch, died on October 13.
The late king was widely seen as a pillar of stability during seven decades of political turmoil in Thailand.
Crown Prince Maha Vajiralongkorn had been expected to become the next king the day after his father’s death, but PM Prayuth Chan-ocha at the time said that he had asked to delay the official proclamation so he could mourn.
Instead, the prince was anointed in a special ceremony 50 days after the death of his father.
The crown prince becomes King Maha Vajiralongkorn Bodindradebayavarangkun, the 10th monarch of the Chakri dynasty, but will also be known as Rama X.
“I would like to accept in order to fulfill his majesty’s wishes and for the benefit of all Thais,” he said in a TV statement.
The broadcast showed one official shuffling on his knees to hand a microphone to the new king in the ceremony at the palace in Bangkok.
Prayuth Chan-ocha, who was among top government figures attending, said the new king would become the “heart and soul” of the nation.
Maha Vajiralongkorn was given the title of crown prince, making him the official heir, in 1972.
He is yet to enjoy the same level of popularity as his father, and spends much of his time abroad.
The prince’s accession to the throne ends a period of uncertainty during which Privy Council President Prem Tinsulanonda, a 96-year-old former prime minister, has been serving as regent.
Thailand’s monarchy is protected from criticism by tough lèse-majesté laws, which restrict media discussion about the royal family’s role, including in the international press.
Most ordinary Thais know only a few details about who the crown prince is and how he lives his life.
In recent years he has tried to improve his profile – important because the king is traditionally seen as a guiding force in Thai politics, which is highly polarized.
Maha Vajiralongkorn’s coronation will not happen until after King Bhumibol’s cremation, expected next year.
Bhumibol’s own coronation took place four years after his brother King Ananda Mahidol died of gunshot wounds in mysterious circumstances.
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