Pisanosaurus (meaning "Pisano lizard") is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton. The fossils were discovered in Argentina's Late Triassic Ischigualasto Formation, which dates to about 228 to 216.5 million years ago.

The exact classification of Pisanosaurus has been the topic of debate by scientists for over 40 years; the current consensus is that Pisanosaurus is the oldest known Ornithischian, part of a diverse group of dinosaurs which lived during nearly the entire span of the Mesozoic Era.

Based on the known fossil elements, Pisanosaurus was a small, lightly built dinosaur approximately 1 m (3 ft 3 in) in length. Its weight was between 2.27–9.1 kg (5–20 lb). These estimates vary due to the incompleteness of the fossil. The tail of Pisanosaurus has been reconstructed as being as long as the rest of the body, based on other early Ornithischians, but as a tail has not been recovered, this is speculative. It was bipedal and, like all other known Ornithischians, was probably herbivorous.

Pisanosaurus mertii was described by Argentine Palaeontologist Rodolfo Casamiquela in 1967. The name Pisanosaurus honors Juan A. Pisano, an Argentine Palaeontologist, while saurus of course is derived from the Greek σαυρος, meaning "lizard". Pisanosaurus is known from a single fragmented skeleton found in Argentina. It is based on a specimen given the designation PVL 2577, which was discovered in the Ischigualasto Formation.