Herrerasaurus (herr-ray-rah-SORE-us) was one of the earliest carnivorous dinosaurs. It lived in what is now Argentina, during the Late Triassic Period (Carnian stage). It is estimated that Herrerasaurus could reach up to 15 feet (5 metres) in length.

It was a bipedal carnivore, which most likely fed on small and medium-sized animals. It had strong hind limbs with short thighs and rather long feet, which means that it was most likely a swift runner.

Herrerasaurus was one of the first dinosaurs and was one of the first animals to embody the distinctive 'carnivorous dinosaur' (theropod-like) shape.

Although Herrerasaurus shared the body shape of the large carnivorous dinosaurs, it lived at a time when dinosaurs were small and insignificant. It was the time of reptiles, not dinosaurs and a major turning point in the Earth's ecology. By the end of the Triassic Period, dinosaurs would rule the planet and the mammal-like reptiles would be nearly gone. At the same time, some of the first mammals began to evolve.

Herrerasaurus lived alongside another early dinosaur (about 230 million years ago), Eoraptor, as well as Saurosuchus, a giant land-living crocodylian, in the jungles of Late Triassic South America. Originally found in the Patagonia region of Argentina, one specimen can be found in the Chicago Field Museum of Natural History.

This dinosaur is an enigmatic creature, showing traits that are found in different groups of dinosaurs. It is also similar to much later Jurassic dinosaurs. Some fossils indicate that Herrerasaurus could grow up to 3 metres in length and 1 metre in height.