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.

The 19-year-old has become a fans' favourite at Aggborough after scoring four goals in 15 appearances.
The National League side have also tied the midfield pair of Ousmane Fane and Elton Ngwatala to new deals.
The French duo, both 22, are contracted until the end of the 2016-17 season.
All three players have helped the Harriers hit their best run of form of the campaign, with the club winning three successive games in February to lift themselves off the bottom of the National League table. Before this month, Kidderminster had won just two of their previous 28 league fixtures.
With 15 matches remaining, the Harriers are seven points from safety.
Chief executive and majority shareholder Colin Gordon has been back in caretaker charge since Dave Hockaday left the club last month.

Harry John Neville Vane, the 11th Baron Barnard, died at Raby Castle near Staindrop on Sunday. He was 92.
In a short statement, his family said he died peacefully surrounded by loved ones.
Lord Barnard, who succeeded his father in 1964, held numerous titles in County Durham. He is survived by five children.
The Raby estate covers large parts of Teesdale including numerous farms and houses which can be recognised by their whitewashed walls.
Raby Castle staff tweeted: "So much thanks for the messages of condolence we've received. Lord Barnard will be sadly missed."
The Vane family have held the Barnard title since 1698.
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An 18-year-old man was left with a serious neck injury after the attack in a Glasgow bar in May.
The man is described as white, tall, with short brown hair and a protruding nose.
At the time he was wearing blue jeans, a white shirt and a khaki green bomber style jacket.
Det Con Nikki McPherson said: "This was a brutal attack that left a man with serious injuries and requiring urgent medical treatment."
She added: "It is important that we trace whoever is responsible as soon as possible.
"I would ask anyone who recognises the man in the CCTV image to contact police as he may be able to assist with our inquiries."
The victim was taken to the Queen Elizabeth University Hospital after the attack at The Argosy bar in Paisley Road West, Cardonald. He has since been discharged
The attack took place during a party at about 23:00 on 13 May.
Det Con McPherson said: "We would also like to hear from anyone who witnessed this incident and is yet to come forward and speak to police, so please do get in touch."
A rare sea turtle found washed up on a Welsh beach has died despite medical treatment at Bristol Aquarium.
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Relegation-threatened Kidderminster Harriers have extended the youth loan of midfielder Ben Whitfield from Premier League side Bournemouth until the end of the season.
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Lord Barnard has died at his County Durham castle, his family has confirmed.
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Police have issued a CCTV image of a man they wish to trace over an assault with a weapon in a pub.