"His Majesty has passed away peacefully at Siriraj Hospital," the palace said in a statement in Thai, adding that the king died at 3.52 pm local time, Channel News Asia reported.
King Bhumibol had spent much of the past year at Bangkok's Siriraj Hospital, where he was treated for various ailments. A palace statement on Wednesday said that the king's blood pressure had dropped, his pulse had quickened, his liver was not functioning normally, and a blood test showed signs of infection.
"Even though the doctors tried their best to save His Majesty, his condition deteriorated," Thursday's statement said.
King Bhumibol was the world’s longest-reigning monarch, reigning over the country for 70 years.
Thais gathered at Siriraj Hospital broke down in tears on hearing the announcement. Many went on their knees and bowed their heads in front of the building where the king had been receiving medical treatment.
On social media, many users shared the King's biography and photographs, with messages such as "I love the King" and "May I be your subject in every life".
King Bhumibol leaves his wife Queen Sirikit, 84, as well as a son, Crown Prince Maha Vajiralongkorn, and three daughters, Princess Ubol Ratana, Princess Maha Chakri Sirindhorn and Princess Chulabhorn Walailak.