Marium the 8-month-old dugong died on Saturday, three days after it was taken from the sea and placed in a nursery tank.
The National Parks, Wildlife and Plant Conservation Department said the dugong died at 12.09am on Saturday in the tank as veterinarians were trying to save her life. Details of her death are expected to be released later.
Marum was moved to the nursery tank on Wednesday out of concern for her safety, with torrential rain and rough seas forecast for the Andaman Sea.
The young dugong fell ill last week after encountering a male dugong while being monitored by vets in the Andaman Sea.
Marium has been cared for by vets on Koh Libong in Trang province since being found on a Krabi beach in April. They planned to fully return her to the wild once she was strong enough.
The department said on Wednesday the vets taking care of her were concerned she would not be strong enough to endure the predicted bad weather, prompting her removal from the sea.
Marium, an Arabic name meaning “lady of the sea”, is the first dugong known to have been cared for by humans in Thailand.