The Kemp's Ridley turtle was found in Aberystwyth's North Beach suffering with hypothermia and septicaemia.
Although the female juvenile seemed to recover for a few days, her condition deteriorated and she died on 25 December.
Bristol Aquarium described the turtle as "incredibly unwell", and in "bad shape" despite medical help.
The post on the aquarium's website added: "We hope that the valuable experience of the past 12 days and knowledge gained from working with this turtle will enable us and others to help future stranded sea turtles."
Kemp's Ridleys live in the Gulf of Mexico, but the juveniles spend their time in the Atlantic, travelling about 6,000 miles on ocean currents until they mature.
In 1985, their population fell to a few hundred nesting females. Since then numbers have risen, with about 10,000 nests laid at Rancho Nuevo, Mexico, annually.
The aquarium has advised that any sea turtles found washed up on the beach should not be put back in the water, but for people to contact British Divers Marine Life Rescue.
A rare sea turtle found washed up on a Welsh beach has died despite medical treatment at Bristol Aquarium.