Huayangosauridae is a family of early, relatively small stegosaurs known from the Middle Jurassic of China, roughly 174 to 163 million years ago, representing one of the earliest branches of the plated-dinosaur lineage. Like their much later relative Stegosaurus, huayangosaurids carried a double row of plates along the back and a spiked tail, but their skulls were shorter and taller in profile, and -- unlike every later stegosaur -- they retained small teeth at the front of the upper jaw, a primitive feature lost in more derived members of the group. Their plates were also notably smaller and thinner than those of later stegosaurs, likely limiting how effectively they could have served a heat-regulating function, one of the proposed roles of stegosaur plates generally.
The family's sole confirmed genus is Huayangosaurus, described from Sichuan Province, China, and it remains one of the most complete and informative early stegosaurs known. As a quadrupedal herbivore retaining several primitive skeletal features alongside the hallmark stegosaur body plan, Huayangosaurus has played an important role in reconstructing how the stegosaur lineage's signature plates, spikes, and posture evolved before the group diversified further in the Late Jurassic.