Lin Hui had lived in an air-conditioned enclosure at Chiang Mai Zoo since 2003 and had been due to return to China in October.
The 21-year-old bear was part of Beijing's "panda diplomacy" programme in northern Thailand, an area plagued by air pollution.
"Her old age caused her blood vessels to deteriorate, and created clots in her blood vessels in several organs, causing her death," said officials at Chiang Mai Zoo.
The beloved panda suffered a nosebleed before slipping into a coma and dying. Chiang Mai Zoo said the bleeding was caused by a "tumour on her nose's blood vessels".
Giant pandas are among the world's most endangered animals and can live up to 30 years in captivity, according to the World Wildlife Fund.
Lin Hui's death prompted an outpouring of grief, with politicians expressing condolences.
Zoo director Wutthichai Muangman said at the time Thailand would have to hand over a 15 million baht ($435,000) insurance payout to China over the death.
Chinese officials said they would also investigate the incident, with Beijing expressing regret the creature could not have been saved.