It's unusual to see leatherbacks in UK waters as they prefer much warmer seas like the Mediterranean or the Caribbean.
But the leathery creature was seen by a man in Alloa who alerted the Scottish SPCA (Society for the Prevention of Cruelty to Animals).
SPCA officers say they've no idea how the 1.65 metre reptile ended up in Scotland, but when the officers tried to find the turtle to return it to the sea, it had disappeared.
A SPCA spokeswoman said: "We really have no idea where it has gone, if it is injured we expect it to come back to the Alloa coast again, if it is not injured it may well be back at sea."
The experts have asked people living along the coast of the Firth of Forth to look out for the turtle.