Thai King Bhumibol Adulyadej, the world's longest-reigning monarch, died Thursday aged 88, according to a royal household statement.
No cause of death was given. The king died peacefully at Siriraj Hospital in Bangkok, the statement read.
Bhumibol was a beloved and deeply revered figure whose seven decades on the throne provided a consistent backdrop of stability to the frequent turbulence of Thai politics.
Revered by Thais as semi-divine, Bhumibol enjoyed near-universal admiration. He ascended to the throne on June 9, 1946.
The country was set to enter a one-year mourning period following the king's death, Prime Minister Prayut Chan-o-cha said in a nationally televised address.
"The prosperous reign of the king has come to an end," Prayut said. "He has touched all of our lives tremendously, and we owe him our gratitude."
"Even though we are in mourning, our beloved country needs to move forward. Do not let this death hold us back from following through with his majesty's vision of prosperity and peace," Prayut added.
Prayut called the king's death the "country's biggest loss" since 1946, when King Rama VIII, Bhumibol's brother, died unexpectedly.
Crown Prince Vajiralongkorn, 64, will be the country's next king, Prayut said, noting that the royal succession process would be carried out in accordance with Thailand's constitution.
Prayut did not mention Vajiralongkorn by name, but he spoke of the "heir who was named in 1972," a likely reference to the crown prince.
Public buildings were asked to lower their flags for 30 days beginning Friday. The public was urged not to hold any form of celebration for 30 days, Prayut said.
Public sector employees were asked to wear black for one year. The public was also encouraged to wear black for the next year, but that suggestion is not mandatory, the premier said.
Thais can hold religious activities and pray according to their own faiths so the king will continue to watch over Thailand and ensure that the peace enjoyed under his reign continues, Prayut added.
Hundreds of the king's supporters gathered outside Siriraj, where he had spent most of the past decade being treated for various ailments.
His last public appearance was in January.