似鸟龙下目（Ornithomimosauria）意为「鸟类模仿者蜥蜴」，是一群兽脚亚目恐龙，外表类似现代的鸵鸟，例如似鸡龙。牠们是群快速、敏捷、杂食性或草食性恐龙，生存于白垩纪的劳亚大陆（现在的亚洲、欧洲、北美洲）。似鸟龙下目首次出现于早白垩纪，并存活到晚白垩纪。牠们似乎与较原始的虚骨龙类恐龙有亲缘关系，例如细颈龙与暴龙科。似鸟龙下目的原始物种包括：似鹈鹕龙、神州龙、似鸟身女妖龙，可能还有大型的恐手龙，恐手龙的手臂长度可达2.4公尺。更先进的种都属于似鸟龙科，包括：似鸡龙、古似鸟龙、似鹅龙、似鸵龙、以及似鸟龙。有些古生物学家，例如保罗·塞里诺（Paul Sereno），认为充满谜题的阿瓦拉慈龙科是似鸟龙下目的近亲，并将牠们分类于似鸟龙超科（Ornithomimoidea）。

The Ornithomimosauria, ornithomimosaurs ("bird-mimic lizards") or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia (now Asia, Europe and North America), as well as Africa and possibly Australia. The group first appeared in the Early Cretaceous and persisted until the Late Cretaceous. Primitive members of the group include Nqwebasaurus, Pelecanimimus, Shenzhousaurus, Hexing and probably the huge Deinocheirus, the arms of which reached 2.4 m (8 feet) in length. More advanced species, members of the family Ornithomimidae, include Gallimimus, Struthiomimus, and Ornithomimus. Some paleontologists, like Paul Sereno, consider the enigmatic alvarezsaurids to be close relatives of the ornithomimosaurs and place them together in the superfamily Ornithomimoidea (see classification below).