Thai Crown Prince Maha Vajiralongkorn, 64, has become the country's new king, succeeding his much-revered late father King Bhumibol Adulyadej.
He accepted the throne in a televised broadcast following an invitation from parliament, formalising his accession.
King Bhumibol, the world's longest-reigning monarch, died on 13 October.
The late king was widely seen as a pillar of stability during seven decades of political turmoil in Thailand.
The crown prince had been expected to become the next king the day after his father's death, but Prime Minister Prayuth Chan-ocha at the time said that he had asked to delay the official proclamation so he could mourn.
Instead, the crown prince was anointed in a special ceremony 50 days after the death of the late Thai king.
He 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 fulfil his majesty's wishes and for the benefit of all Thais,'' he said in a televised 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.
Mr Prayuth, 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.
His 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 lese-majeste 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 polarised.
His 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.