Xiaosaurus ("dawn lizard", /ʃaʊˈsɔːrəs/), is a genus of small herbivorous dinosaur from the middle Jurassic, approximately 170.3 to 163.5 mya.

In 1983 Dong Zhiming and Tang Zilu named the fossils under the type species Xiaosaurus dashanpensis.

[1] The holotype, IVPP V6730A, was found in the lower Xiashaximiao Formation of which the age is uncertain: both the Bajocian and the Bathonian–Callovian have been proposed.

Xiaosaurus has sometimes been considered a nomen dubium and an ornithischian of uncertain affinities, possibly a basal cerapod or marginocephalian.

However, Paul Barrett et al. in 2005 concluded it to be provisionally valid, as it possessed a single unique derived trait or autapomorphy: a mediolaterally (seen from the front) straight humerus.