Herrerasaurus is likely a genus of saurischian dinosaur from the Late Triassic period. Measuring 6 m (20 ft) long and weighing around 350 kg (770 lb), this genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 in South America. All known fossils of this carnivore have been discovered in the Ischigualasto Formation of Carnian age (late Triassic according to the ICS, dated to 231.4 million years ago) in northwestern Argentina. The type species, Herrerasaurus ischigualastensis, was described by Osvaldo Reig in 1963 and is the only species assigned to the genus. Ischisaurus and Frenguellisaurus are synonyms.

For many years, the classification of Herrerasaurus was unclear because it was known from very fragmentary remains. It was hypothesized to be a basal theropod, a basal sauropodomorph, a basal saurischian, or not a dinosaur at all but another type of archosaur. However, with the discovery of an almost complete skeleton and skull in 1988, Herrerasaurus has been classified as an early saurischian in most of the phylogenies on the origin and early evolution of dinosaurs. It is a member of the Herrerasauridae, a family of similar genera that were among the earliest of the dinosaurian evolutionary radiation.

Herrerasaurus was named by paleontologist Osvaldo Reig after Victorino Herrera, an Andean goatherd who first noticed its fossils in outcrops near the city of San Juan, Argentina in 1959. These rocks, which later yielded Eoraptor, are part of the Ischigualasto Formation and date from the late Carnian stage of the Late Triassic period. Reig named a second dinosaur from these rocks in the same publication as Herrerasaurus; this dinosaur, Ischisaurus cattoi, is now considered a junior synonym and a juvenile of Herrerasaurus.

Reig believed Herrerasaurus was an early example of a carnosaur, but this was the subject of much debate over the next 30 years, and the genus was variously classified during that time. In 1970, Steel classified Herrerasaurus as a prosauropod. In 1972, Peter Galton classified the genus as not diagnosable beyond Saurischia. Later, using cladistic analysis, some researchers put Herrerasaurus and Staurikosaurus at the base of the dinosaur tree before the separation between ornithischians and saurischians. Several researchers classified the remains as non-dinosaurian.

Two other partial skeletons with skull material were named Frenguellisaurus ischigualastensis by Fernando Novas in 1986, but this species, too, is now thought to be a synonym. Frenguellisaurus ischigualastensis was discovered in 1975 and was described by Novas (1986), who considered it a primitive saurischian and possibly a theropod. Novas (1992) and Sereno and Novas (1992) examined the Frenguellisaurus remains and found them referable to Herrerasaurus. Ischisaurus cattoi was discovered in 1960 and described by Reig in 1963. Novas (1992) and Sereno and Novas (1992) reviewed its remains and found them also to be referable to Herrerasaurus.

A complete Herrerasaurus skull was found in 1988 by a team of paleontologists led by Paul Sereno. Based on the new fossils, authors such as Thomas Holtz and José Bonaparte classified Herrerasaurus at the base of the saurischian tree before the divergence between prosauropods and theropods. However, Sereno favored classifying Herrerasaurus (and the Herrerasauridae) as primitive theropods. These two classifications have become the most persistent, with Rauhut (2003) and Bittencourt and Kellner (2004) favoring the early theropod hypothesis, and Max Langer (2004), Langer and Benton (2006), and Randall Irmis and his coauthors (2007) favoring the basal saurischian hypothesis. If Herrerasaurus were indeed a theropod, it would indicate that theropods, sauropodomorphs, and ornithischians diverged even earlier than herrerasaurids, before the middle Carnian, and that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum". This view is further supported by ichnological records showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur. These footprints date from the early Carnian Los Rastros Formation in Argentina, which predates Herrerasaurus by several million years.

The study of early dinosaurs such as Herrerasaurus and Eoraptor, therefore, has important implications for the concept of dinosaurs as a monophyletic group (a group descended from a common ancestor). The monophyly of dinosaurs was explicitly proposed in the 1970s by Galton and Robert T. Bakker, who compiled a list of cranial and postcranial synapomorphies (common anatomical traits derived from the common ancestor). Later authors proposed additional synapomorphies. An extensive study of Herrerasaurus by Sereno in 1992 suggested that of these proposed synapomorphies, only one cranial and seven postcranial features were actually derived from a common ancestor and that the others were attributable to convergent evolution. Sereno's analysis of Herrerasaurus also led him to propose several new dinosaurian synapomorphies.

Herrerasaurus was a lightly built bipedal carnivore with a long tail and a relatively small head. Adults had skulls up to 56 cm (22 in) long and were up to 6 m (20 ft) in total length and 350 kg (770 lb) in weight. Smaller specimens were about 4.5 m (15 ft) long and weighed about 200 kg (440 lb).