The male pilot whale was found struggling in a canal on Monday in the southern province of Songkhla.
Rescuers tried for five days to keep it alive but couldn’t save it.
A post mortem found 80 pieces of plastic in inside it, weighing eight kilograms.
Komchai Thanapanich, from the environmental group Thai Whales, took part in the attempted rescue and told Sky News that before its death the whale vomited up five plastic bags.
She believes the Thai government needs to help cut the country’s use of plastic.
“The government should have a policy that when you want to take a plastic bag you should pay – it will help a lot,” she said.
Labs in Southern Thailand will now do further tests on the whale’s tissue to find out why it died.
Its bones are being tested by experts in Phuket.
It is believed the animal may also have been suffering for other health problems before it starved to death.
Dr Watchara Sakornwimon, the vet that carried out the post mortem, explained that the plastic in its stomach meant it couldn’t eat.
She said: “The food cannot go through the intestines so the animal cannot get any nutrients – and some plastics degrade or are in small pieces already so it may be contaminating the body.
“The plastic has additives which can be toxic for the animal.”
Thailand uses a huge amount of single use plastic, with the government recently announcing it was considering introducing a levy on carrier bags.
Jatuporn Buruspat, head of the Marine and Coastal Resources Department, said they planned to raise public awareness of the problem on World Oceans Day on 8 June and call for reduced use of plastic.
“We will use the whale case and invite all sectors to show their intentions on how to reduce the use of plastic in Thailand,” he told Reuters.
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It is estimated globally 8 million tonnes of plastic are clogging up the world’s oceans.
:: Sky’s Ocean Rescue campaign encourages people to reduce their single-use plastics. You can find out more about the campaign and how to get involved at www.skyoceanrescue.com