Huayangosaurus (hoy-YANG-oh-SORE-us) was a stegosaurian from Middle Jurassic in China. It derives its name from "Huayang" an alternate name for Sichuan, the province it was discovered.

It lived 165 million years ago, some 20 million years before its famous relative, Stegosaurus appeared in North America. At only 4.5 metres long, it was also much smaller than its famous cousin.

Found in the Lower Shaximiao Formation, Huayangosaurus shared the local Middle Jurassic landscape with the sauropods Shunosaurus, Datousaurus, Omeisaurus and Protognathus, the ornithopod Xiaosaurus and the carnivorous Gasosaurus.

Huayangosaurus remains from twelve inidvidual animals were recovered from the Dashanpu Quarry near Zigong in Sichuan, and named by Dong Zhiming.

As it is the most basal stegosaurian, it is placed in within its own family 'Huayangosauridae. It is also morphologically distinct from later (stegosaurid) forms. Its skull was broader and had premaxillary teeth in the front of its mouth. All later stegosaurians lost these teeth.

Like many other stegosaurians it had plates all down its back, and spikes on its tail. Two large spikes where above its hips, andh may have been used for deterring an attack from above (considering as it was a fairly short in height compared to later stegosaurians). Its plates were smaller than those of Stegosaurus, with much less surface area. Thus they would have been much less effective heat regulators, one of the postulated functions of plates.