角龙亚目（Ceratopsia或Ceratopia）意为“有角的面孔”，是群草食性、喙状嘴恐龙，生存于白垩纪的北美洲与亚洲。牠们的祖先出现于侏㑩纪晚期。已知最早的角龙类是隐龙，生存于1亿6120万到1亿5570万年前。角龙类在白垩纪-第三纪灭绝事件灭绝，约6550万年前。

早期物种是小型、二足恐龙，例如鹦鹉嘴龙。较晚期的物种，包括角龙科的尖角龙与三角龙，演化为非常大的四足恐龙，脸部发展出延长的角状物，以及颈部头盾。头盾可能用来保护容易受伤的颈部被掠食动物攻击，也可能作为展示物、调节体温、大型颈部与咀嚼肌肉附着用、或者是综合以上几点。角龙类在体型上的变化从1公尺到9公尺，体重的变化从23公斤到5.4吨。

Ceratopsia or Ceratopia (/ˌsɛrəˈtɒpsiə/ or /ˌsɛrəˈtoʊpiə/; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7 million years ago. The last ceratopsian species, Triceratops prorsus, became extinct during the Cretaceous–Paleogene extinction event, 66 million years ago.