His Majesty King Bhumibol Adulyadej passed away at Siriraj Hospital at 3.52pm on Thursday, the Royal Household Bureau announced.
His Majesty passed away peacefully at Chalermphrakiet Building at Siriraj Hospital in Bangkok it was announced in a national broadcast.
All royal family members were in attendance at the time.
His Majesty was 88 and would have turned 89 on Dec 5 this year.
The ninth King of the Chakri Dynasty ascended to the throne on June 9, 1946 and was the world's longest-reigning monarch. People across the country celebrated the 70th year of his accession to the throne this year.