厚头龙下目（Pachycephalosauria）又称肿头龙下目，希腊文意思为「厚头蜥蜴」，是鸟臀目恐龙的一个演化支。大多数已知的厚头龙类生存在晚白垩纪的北美洲与亚洲。厚头龙类全是二足、草食性/杂食性恐龙，并具有厚头颅骨。有些物种的头颅骨呈圆丘状，顶部有数吋厚，其他的头部则是呈平坦或楔状。颅顶可能环绕者骨瘤（例如：厚头龙）、尖刺（冥河龙）、或是两者兼有（龙王龙）。在传统的分类法中，这些不同形状的颅顶，代表不同的属、科。但近年的研究多认为，不同形状的颅顶代表者圆颅顶厚头龙类的不同生长阶段。

目前已知最早的厚头龙类包括：生存于侏㑩纪中期吉尔吉斯的齿饰费尔干纳头龙、白垩纪早期德国的狭盘龙，但罗伯特·苏利文（Robert Sullivan）在2006年对这两种是否为厚头龙类提出质疑。

Pachycephalosauria (/ˌpækiˌsɛfələˈsɔːriə, ˌkɛ-/; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some fossils, the skull roof is domed and several centimeters thick; in others it is flat or wedge-shaped. While the flat-headed pachycephalosaurs are traditionally regarded as distinct species or even families, they may actually represent juveniles of dome-headed adults. The domes were often surrounded by nodes and/or spikes.