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.
The department said she died of shock and other reasons, including several pieces of plastic found in her intestine. More details of her death will be released later.
Mariam was moved to the nursery tank on Wednesday out of concern for her safety, amid torrential rain and rough seas forecast for the Andaman Sea.
The female dugong fell ill last week after encountering a male dugong while being monitored by vets in the Andaman Sea.
Mariam was 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.