Staff at the Galapagos National Park in Ecuador say Lonesome George, a giant tortoise believed to be the last of its subspecies, has died.
Scientists estimate he was about 100 years old.
Park officials said they would carry out a post-mortem to determine the cause of his death.
With no offspring and no known individuals from his subspecies left, Lonesome George became known as the rarest creature in the world.
For decades, environmentalists unsuccessfully tried to get the Pinta Island tortoise to reproduce with females from a similar subspecies on the Galapagos Islands.