Lesothosaurus (Le-SO-toe-sore-us) is a member of the herbivorous clade of dinosaurs, the Ornithischia. It was named by paleontologist Peter M. Galton in 1978, the name meaning "lizard from Lesotho".

The genus is monotypic, meaning there is only one valid species, Lesothosaurus diagnosticus, within the genus. Lesothosaurus was originally considered an ornithopod. However, more recent work by Paul Sereno has suggested that it may actually represent one of the most primitive of all known ornithischian dinosaurs.

The taxonomic history of Lesothosaurus is complex and it has long been confused with Fabrosaurus, another small ornithischian from the same locality.

Lesothosaurus was a small (one metre in length), bipedal plant-eater. Skeletal remains suggest that it was a fast runner. The small skull was short and flat, with large eye sockets. The hind limbs of Lesothosaurus were much longer than the fore limbs, which were quite short with small 'hands'.

Lesothosaurus lived in the hot, arid conditions of Lesotho and South Africa, during the Early Jurassic. Remains of Lesothosaurus have been collected from the Upper Elliot Formation, dating it to the Hettangian-Sinemurian portions of the Early Jurassic.