The 2.75m-long, 100-year-old creature was found near Harlech in 1988, more than 4,700 miles from where it was born in the West Indies.
Scientists using satellites to track leatherbacks found the beasts spend most of their lives in northern waters.
They venture so far from home to eat their favourite food - jellyfish.
More and more of the beasts have been spotted in British waters, and many experts now want them to be recognised as a British species.
Turtle numbers have been dropping because of building near their nest sites, egg snatching, pollution and fishing.
About 50,000 leatherbacks are killed every year by long-line fishing, when they are accidentally caught on fish hooks.
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