Riojasaurus (meaning "lizard from La Rioja") was a herbivorous sauropodomorph dinosaur named after La Rioja Province in Argentina where it was found in the Los Colorados Formation in the Ischigualasto-Villa Unión Basin by José Bonaparte.

It lived during the Late Triassic (Norian stage) and grew to about 6.6 metres (22 ft) long.

Riojasaurus incertus was named by Bonaparte (1969) and was based on the holotype specimen, PVL 3808, which was discovered in 1966 and consists of a postcranial skeleton which lacks the skull and mandibles, but preserves 6 presacral vertebrae, diverse caudals, both scapulae, the ischia, and the bones of the hand.

Comparisons between the scleral rings of Riojasaurus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.

However, studies at Bristol University, England, suggest that it is unique in some key ways, such as the longer bones in its neck.