The highly-revered King Bhumibol Adulyadej, the world's longest-reigning monarch, passed away at the age of 88 in Bangkok on Thursday.
He became king in 1946 after the still unexplained gunshot death of his elder brother, 20-year-old King Ananda Mahidol who was also known as Rama VIII.
"Even though the board of doctors has closely monitored and treated him to the best of its abilities, the king's condition never improved", the palace said in a statement.
King Bhumibol, who was born in Cambridge in the U.S. state of MA, acceded to the throne on 9 June 1946 after his brother, King Ananda Mahidol, died.
His most noteworthy appearance on the political stage came in May 1992, amid violence between pro-democracy campaigners and the army.
Thais have had many years to get used to the prospect of no longer having Bhumibol as their king, he has not been seen by the public for a year.
As well as the crown prince and his wife, Queen Sirikit, Bhumibol is survived by three other children: Ubolratana, Chulabhorn and Maha Chakri Sirindhorn.
Bhumibol and Sirikit Wednesday in 1950, a week before the king's coronation ceremony.
"I'm waiting for a miracle to bring the king back to life because he said he wanted to live until 120 years old".
After mass protests against Prime Minister Thaksin Shinawatra began in 2006, Bhumibol urged the country's top courts to resolve the political crisis.
Framed posters, paintings and photographs of the king are ubiquitous in Thai homes and shops, depicting not only an exalted figure in glimmering robes, but also an ordinary-looking man with a camera strapped around his chest.
The military has deep links with the palace and many inside the kingdom saw the putsch as a move to ensure generals could stamp down on any instability during a succession.
Kerry said in a statement Thursday, "Our thoughts and prayers are with you". He recalled meeting the king during a 2012 visit to Thailand.
A woman wept at the announcement of King Bhumibol Adulyadej's death.
Thailand has lese-majeste laws that impose long prison terms for insulting the monarchy.
The loss of the king, who is practically deified in Thailand, casts uncertainty on the country's future.
King Bhumibol will be a hard act to follow.
According to the New York Times, his only son and heir-apparent - Crown Prince Vajiralongkorn - "is seen by many as a jet-setting playboy and not held in the same regard as his father".