Chuang Chuang collapsed on Monday in his enclosure following a meal of bamboo leaves, according to Wutthichai Muangmun, zoo director in the northern Thai city of Chiang Mai.
Staff at the zoo will wear black and white to mourn the giant panda, who was on a long-term loan from China.
No sign of illness or injury was found on the panda’s body, and he had recently passed a health check-up.
Chuang Chuang was loaned to Thailand from China in 2003, with his female mate Hui. The original 10-year loan plan was extended for another 10 years.
While the loan was for research and conservation purposes, it was also seen as an act of friendship by China, which has sent pandas to other countries.
The two pandas were a huge hit and were stars of a 24-hour TV channel which was dedicated to them for three years.
According to local media reports, fans of the panda have posted messages online, thanking Chuang Chuang for the happiness he had brought to Thailand.
Kannikar Nimtrakul, the Chiang Mai Zoo’s vet, said giant pandas have a life expectancy of 14-20 years.
In 2007, Chuang Chuang was encouraged to mate with Lin Hui, with keepers showing him videos of other pandas mating.
While Lin Hui did have a baby, Lin Ping in 2009, she became pregnant through artificial insemination.
The zoo said 15 million visitors had come to see the panda family in the last 16 years, producing about 295m baht (£7.7m) in income.
It may take some time to discover the cause of death, as no post-mortem can be carried out without a Chinese expert, under an agreement with the zoo and the Chinese government.
Commemorative events for Chuang Chuang will take place at the zoo, where a minute’s silence has already taken place.