Riojasaurus (ree-OH-ha-SAWR-us) was a prosauropod plant-eating dinosaur named after La Rioja Province in Argentina where it was found by José Bonaparte.

It lived during the Late Triassic and Early Jurassic.

It grew to about 11 metres long, had a heavy body, bulky legs and a long neck and tail. It probably moved slowly on all fours and was unable to rear up on its back legs. Experts think that Riojasaurus probably travelled in herds for protection. Unlike smaller prosauropods such as Plateosaurus, Riojasaurus could not rear up on its hind legs. It trundled along on four heavy limbs, unable to make a quick getaway from carnivores like Saurosuchus.

No skull was found with the first skeleton of Riojasaurus, but its long slender neck suggests that its head was quite small. Riojasaurus probably fed on the foliage of tall conifers and monkeypuzzle trees, raking off leaves and needles with its weak, spoonshaped teeth. Huge Riojasaurus is the only known melanosaurid to live in South America.

This dinosaur was omnivorous. It was active during the omnivorous age about 221-210 million years ago.