Orphaned dugong became social media star after she was rescued and hand-reared by biologists.
The female dugong - a large ocean mammal - was named "Marium", which means "lady of the sea", and was nurtured by marine experts after she was found orphaned on a beach in the south of the country in April.
The young creature became an internet hit after images of biologists embracing and feeding her with milk and grass spread across social media. She was dubbed the "nation's sweetheart" by Thailand's Department of Marine and Coastal Resources (DMCR).
Veterinarians and volunteers had set out in canoes to feed Marium for up to 15 times a day while also giving her health checks.
Last week, she was found bruised after being chased and supposedly attacked by a male dugong during the mating season, said Jatuporn Buruspat, the director-general of the DMCR.
She was brought in for treatment in the artificial sea on Libong Island in Krabi province.
"We assume she wandered off too far from her natural habitat and was chased, and eventually attacked by another male dugong, or dugongs, as they feel attracted to her," he said on Saturday.
An autopsy showed a large amount of plastic waste in her intestine, which could also have played a part in her death as it led to gastritis and blood infection, he said.
"She must've thought these plastics were edible," Jatuporn said.
The dugong is a species of marine mammal similar to the manatee and can grow to about 3.4 meters in length.
According to National Geographic, it can be found in warm coastal waters from East Africa to Australia, including the Red Sea, the Indian Ocean, and the Pacific Ocean.
Some believe that the dugong was the inspiration for ancient maritime tales of mermaids and sirens.
Today, its conservation status is listed as vulnerable by the International Union for Conservation of Nature, meaning it is at high risk of extinction in the wild.
The slow-moving animals are an easy target for coastal hunters and were sought for their meat, oil, skin, bones, and teeth. Dugongs now have protected status throughout their range but remain at risk from pollution, climate change, development, and increased shipping.
In June, an association of 10 Southeast Asian countries, including Thailand, adopted a joint declaration to combat marine debris in the region.