The pilot whale was discovered on Monday in a canal in the southern province of Songkhla and received treatment from a team of veterinarians.
The whale spit out five plastic bags on Friday and later died, the Marine and Coastal Resources Department said on its website.
An autopsy found another 80 bags and other plastic items weighing eight kg in the whale's stomach.
Jatuporn Buruspat, head of the department, said the whale probably thought the floating plastic bags were food.
Pilot whales mainly feed on squid, but are known to eat octopus and small fish when squid are not available, according to the American Cetacean Society, a whale conservation group.
Thais use huge numbers of plastic bags but authorities have launched campaigns to try to encourage people to use fewer and to introduce reusable bags.
Globally, eight million tonnes of plastic - bottles, packaging and other waste - are dumped into the ocean every year, killing marine life and entering the human food chain, the United Nations Environment Programme said in December.
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