ऑर्नीथिस्किया (Ornithischia) डायनासोरों के दो मुख्य प्रकारों में से एक है । जीववैज्ञानिक वर्गीकरण की दृष्टि से यह एक क्लेड माना जाता है। इसकी सभी सदस्य जातियाँ विलुप्त हो चुकी हैं। "ऑर्नीथिस्किया" का अर्थ "पक्षी की कमर वाला प्राणी" है, हालांकि पक्षी इसी क्लेड का भाग न होकर सॉरिस्किया क्लेड का भाग हैं।

Ornithischia is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name Ornithischia, or "bird-hipped", reflects this similarity and is derived from the Greek stem ornith- (ὀρνιθ-), meaning "bird", and ischion (ἴσχιον), meaning "hip". However, birds are only distantly related to this group as birds are theropod dinosaurs. Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs, the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, and the ornithopods. There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. Some were at least partially covered in filamentous pelts, and there is much debate over whether these filaments found in specimens of Tianyulong, Psittacosaurus, and Kulindadromeus may have been primitive feathers.