Once one has seen a mata mata, it is unlikely to ever be confused with any other turtle. A sidenecked (Pleurodira) turtle from South America, the mata mata is truly one of a kind. The word mata mata can be translated from spanish to "it kills, it kills", its binomial name is chelus fimbriatus. Brown or black oblong carapace to 44.9 cm. The plastron is reduced, narrowed, hingeless, shortened towards the front and deeply notched at the rear. Bridges are narrow. Plastron and bridge are cream to yellow or brown.

The head is very distinctive, triangular, large, extremely flattened. There are numerous tubercles and flaps of skin. There are two barbels on the chin and two additional filamentous barbels at the jaw. The snout is long and tubular. The upper jaw is neither hooked nor notched.

Head, neck, tail, and limbs are grayish brown on adults. The neck is very long, longer than the vertebra under the carapace, and is fringed with small skin flaps along both sides.

Each forefoot has five webbed claws. Males have concave plastrons and longer, thicker tails.