Prachuab Mokarat, director of the Trang marine and coastal resources centre, said the male dolphin, about 15-20 years old, was badly bruised on the head.
He said the dolphin might have died one day earlier. Weighing 170 kg, the dolphin was 2.20 metres long.
Tawan Tui-on, chairman of the Trang Dolphin Conservation Club, said the most recent survey in the Trang sea found at least 100 dolphins.
They mainly die during the monsoon season, while some are caught in fishing nets and are killed rather than released, he said, urging the authorities to find better measures to protect the animals.