It is the first sighting of the creature in English waters this year.
Marine scientists say it is unusually early in the year for the turtles, which are an endangered species, to enter Cornish waters.
Newquay resident Jo Leach spotted the turtle while out on her uncle's crab-potting boat five miles south of Mevagissey.
The Marine Conservation Society said it was unusually early for the species to be sighted as it does not normally make an appearance until the summer.
The turtle was only seen 23 times in
UK waters in 2003.
The critically endangered leatherback is the largest marine turtle species, measuring up to 9 feet 6 inches (2.91 metres) in length and weighing up to 2,015lb (916 kilograms).
Jo Leach said: "My uncle was using binoculars to look out for the fin whales when he saw something that he thought looked like a rock breaking the surface.
"We approached the object and when we were about 15 feet away we could see it was a leatherback turtle.
"It gazed at us for a few minutes and then slowly swam off, as if it had had enough of looking at us."
The leatherback sighting this week is the second record for the species this year. A dead leatherback washed ashore at Crail in Fife, Scotland, at the end of January.
Peter Richardson, species policy officer with the Marine Conservation Society (MCS), said: "The sighting is unusual in that it occurred so early in the year.
"Live leatherbacks usually start appearing around about May and are spotted throughout the summer, peaking in August and tailing off in November.
"This sighting, coupled with the incredible sighting of 70 adult basking sharks seen together three miles north of Guernsey on February 14, suggests that conditions in UK seas are unseasonably good for feeding basking sharks and leatherbacks."
Last year saw only 23 live sightings of leatherback turtles were recorded.