Riojasaurus is a genus of dinosaur belonging to the family Melanorosauridae. It lived approximately 230 to 220 million years ago during the Late Triassic period in what is now the Ischigualasto Formation of Argentina. The first fossils of Riojasaurus were discovered in the 1960s and 1970s. Riojasaurus is the only known riojasaurid to live in South America.

Riojasaurus was a large-sized dinosaur, reaching lengths of up to 10 meters and weighing around 1 to 2 tons. It had a relatively long neck, a robust body, and a long tail that provided balance. Its hind limbs were longer and more strong than its forelimbs, suggesting a bipedal stance with the ability to adopt a quadrupedal posture. Nevertheless, the shape and structure of the hand and shoulder girdle have been interpreted as incompatible with a quadrupedal gait. It had leaf-shaped teeth designed for cropping vegetation.

Riojasaurus likely exhibited a primarily quadrupedal stance while feeding, utilizing its robust hind limbs and tail for stability. It may have traveled in herds, offering protection against predators and facilitating foraging efficiency. Its leaf-shaped teeth suggest a selective feeding strategy, allowing it to process plant material effectively and extract nutrients. The upper jaw housed 5 front teeth, followed by an additional row of 24 teeth that extended behind them and ended beneath the eyes.