The Chaco tortoise (Chelonoidis chilensis), also known commonly as the Argentine tortoise, the Patagonian tortoise, or the southern wood tortoise, is a species of tortoise in the family Testudinidae. The species is endemic to South America.
The carapace of C. chilensis can measure up to 43.3 cm (but usually less than 25 cm) in a straight line, and may be either totally yellowish brown or have dark-brown to black rings surrounding a tan center on each scute. Specimens found farther south tend to be much larger than those found in farther north populations. The rim of the shell is slightly serrated and has a dark wedge of pigment at the back edge of each scute. The plastron may be uniformly yellowish-brown or have a dark triangular wedge along the seams of each scute. The head, limbs and tail are greyish to yellowish-brown, with the front of each forelimb covered with large, angular scales and each thigh featuring several enlarged tubercles.