鸟脚亚目（Ornithopoda）是鸟臀目恐龙的一个演化支，牠们起初是小型、二足、快速奔跑的草食性动物，后来种类与体型都逐渐变大、变多，直到牠们成为白垩纪时代里最成功的草食性恐龙之一，并成为北美洲的优势物种。鸟脚类恐龙在演化上最主要的优点是咀嚼方式的进展，是爬行动物里最复杂的咀嚼方式，可与现代哺乳类相比，例如牛。牠们在鸭嘴龙科时达到顶峰，而与其他恐龙一起在白垩纪-第三纪灭绝事件中灭绝。所有大陆都发现了牠们的踪影，虽然在南极洲发现的化石还未命名，而牠们在南半球通常是罕见的。

Ornithopods (/ɔːrˈnɪθəˌpɒdz, ˈɔːrnɪ-/) or members of the clade Ornithopoda (/ˌɔːrnɪˈθɒpədə/ or /ɔːrˌnɪθəˈpoʊdə, ˌɔːrnɪ-/) are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American landscape. Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a non-avian dinosaur, rivaling that of modern mammals such as the domestic cow. They reached their apex in the duck-bills (hadrosaurs), before they were wiped out by the Cretaceous–Paleogene extinction event along with all other non-avian dinosaurs. Members are known from all seven continents, though they are generally rare in the Southern Hemisphere.