The animal was found dead in his corral early Sunday at the Tortoise Breeding and Rearing Center in Puerto Ayora on Santa Cruz Island, the Galapagos National Park Service reported.
Park ranger Fausto Llerena, his longtime keeper, found George stretched out toward his watering hole but showing no signs of life.
The body of the animal, which had been the sole living example of the Pinta Island tortoise (Chelonoidis abingdoni), was being held in a refrigerated chamber to avoid decomposition prior to a necropsy, officials said.
Pinta tortoises were thought to be extinct when George was discovered in 1972.
Efforts to get George to reproduce were unsuccessful, as females of a related species put with George produced infertile eggs.
The animal's exact age was unknown but was estimated to be about 100 years, officials said.