Thailand's King Bhumibol Adulyadej, the world's longest-reigning monarch, has died after 70 years on the throne, the palace says.
The king, 88, was widely revered in Thailand. He was seen as a stabilising figure in a country hit by cycles of political turmoil and multiple coups.
He had been in poor health in recent years, making few public appearances.
Crown Prince Maha Vajiralongkorn will be the new monarch, the prime minister has said.
In a televised address to the nation, Prayut Chan-ocha said Thailand would hold a one-year mourning period and that all entertainment functions must be "toned down" for a month.
"He is now in heaven and may be looking over Thai citizens from there," he said of King Bhumibol.
The king's death comes as Thailand remains under military rule following a coup in 2014.
"His Majesty has passed away at Siriraj Hospital peacefully," the palace statement said, adding he had died at 15:52 (08:52 GMT).
Parliament is to hold a special session at 21:00 local time (14:00 GMT).
Crown Prince Vajiralongkorn, who is 63, is much less well known to Thais and has yet to attain his father's widespread popularity. He spends much of his time overseas, especially in Germany
Strict lese-majeste laws mean public discussion of the succession are punishable by lengthy jail terms.
Given the pivotal role the king has played in maintaining the balance of power in Thailand's volatile political environment, the succession will be a formidable challenge for the government, says the BBC's Jonathan Head in Bangkok.
King Bhumibol, who was born in Cambridge in the US state of Massachusetts, acceded to the throne on 9 June 1946 after his brother, King Ananda Mahidol, died.
Though a constitutional monarch with limited powers, King Bhumibol was regarded by many Thais as semi-divine. They looked to him to intervene in times of high tension.
He was seen as a unifying and calming influence through numerous coups and 20 constitutions.
However, his critics argued he had endorsed military takeovers and at times had failed to speak out against human rights abuses.