The panda was put under deep anaesthesia to take CT scans earlier on Saturday, and the team decided to “let Tuan Tuan continue to sleep” after the results indicated his condition was “irreversible”, and that he could no longer “live a quality life”, zoo officials said.
“It would have been extremely painful and risky for Tuan Tuan to resuscitate him from the anaesthesia,” said spokesman Eric Tsao.
Vets first noticed Tuan Tuan, 18, was ill in August, when he began suffering seizures and appeared increasingly unsteady and lethargic.
Medical scans showed he had a brain lesion and he was placed on anti-seizure medication.
The zoo suspected Tuan Tuan had a brain tumour and he was moved into palliative care last month.
The seizures had returned in the past few days, more frequent than before, and medicine could not ease the symptoms.
Fans mourned Tuan Tuan’s passing on social media, while Taipei mayor Ko Wen-je thanked the panda for “bringing happiness to Taiwanese people and making Taipei Zoo more wonderful” in an Instagram post.
The panda couple – whose names combined mean “reunion” or “unity” – had become huge stars in Taiwan since being gifted, and Yuan Yuan has since given birth to two female cubs.
China has long deployed “panda diplomacy”, and the gift of Tuan Tuan and Yuan Yuan was a move seen to endorse the presidency of then Beijing-friendly leader Ma Ying-jeou.
Beijing only loans pandas to foreign zoos, which must usually return any offspring within a few years of their birth to join the country’s breeding programme.
But Taiwan was granted an exception as part of a brief charm offensive China launched in the late 2000s, and was fully gifted both Tuan Tuan and Yuan Yuan and any offspring they had.
Relations between China and Taiwan have been on ice since 2016 with Beijing severing official communications and government visits between the two sides scrapped.