Beipiaosaurus /ˌbeɪpjaʊˈsɔːrəs/ is a genus of therizinosauroid theropod dinosaurs that lived in China during the Early Cretaceous in the Yixian Formation. The first remains were found in 1996 and formally described in 1999. Before the discovery of Yutyrannus, Beipiaosaurus were among the heaviest dinosaurs known from direct evidence to be feathered. Beipiaosaurus is known from three reported specimens. Numerous impressions of feather structures were preserved that allowed researchers to determine the feathering color which turned out to be brownish.

They were relatively small-sized therizinosaurs, measuring 2.2 m (7.2 ft) long and weighing about 27 kg (60 lb) in contrast to the advanced and giant Segnosaurus or Therizinosaurus. The necks of Beipiaosaurus were shorter than in most therizinosaurs, whose are characterized by elongated necks adapted for high-browsing. Also, their feet configuration differs from therizinosaurids, having a generic three-toed pes instead of four as seen in other members.

The exact classification of therizinosaurs had in the past been hotly debated, since their prosauropod-like teeth and body structure indicate that they were generally herbivorous, unlike typical theropods. Beipiaosaurus, being considered to be a primitive therizinosauroid, has features which suggest that all therizinosauroids, including the more derived Therizinosauridae, to be coelurosaurian theropods, not sauropodomorph or ornithischian relatives as once believed.

In 1996, a farmer, Li Yinxian discovered a partial skeleton of a theropod dinosaur near the village of Sihetun. The following year, it was confirmed to have come from the lower beds of the Yixian Formation and represented a single individual. On May 27, 1999, the discovery was announced in the academic journal Nature and the type species Beipiaosaurus inexpectus named and described by Xu Xing, Tang Zhilu and Wang Xiaolin. The generic name Beipiaosaurus translates as "Beipiao lizard" after Beipiao, a city in China near the location of its discovery. Beipiaosaurus is known from a single species, B. inexpectus, the specific name, meaning "unexpected" in Latin, referring to the "surprising features in these animals".

The holotype (type specimen) of Beipiaosaurus inexpectus, IVPP V11559, was recovered in the Jianshangou Beds of the Yixian Formation in Liaoning Province, China. The specimen was collected in sediment deposited during the Aptian stage of the Early Cretaceous period, approximately 125 million years ago. It is housed in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology, in Beijing, China. It consists of a partial, sub-adult, skeleton that is largely disarticulated. A significant number of fossilized bones were recovered, including: cranial fragments, a mandible, teeth, three cervical vertebrae, four dorsal vertebrae, four dorsal ribs, two sacral vertebrae, twenty-five caudal vertebrae with a pygostyle, three chevrons, an incomplete furcula and scapula, both coracoids, both forelimbs, both ilia, an incomplete pubis, an incomplete ischium, a femur, both tibiae (one incomplete), an incomplete fibula, the astragalus and calcaneum, several tarsals, metatarsals, manual and pedal unguals, and skin impressions of the primitive plumage. The pelvic girdle and caudal vertebrae were discovered during a re-excavation of the fossil quarry where the first elements of the holotype were found. These rediscovered elements helped to complete the holotype specimen.

A second specimen, STM 31-1, a partial skeleton, was described by Xu et al. 2009, which preserves a significant covering of unique, elongated feathers. This specimen consisted of a complete skull, a sclerotic ring, the mandible, the atlas and axis bones, nine additional cervical vertebrae, dorsal vertebrae, seventeen cervical ribs, twelve dorsal ribs, both scapulae and coracoids, one complete humerus and proximal humerus, one complete radius and distal radius, one complete ulna and distal ulna, carpals, and some metacarpals. The rear of the skull of this specimen was badly crushed. Li et al. 2014 mentioned a third specimen labelled under the number BMNHC PH000911. This specimen hails from the Sihetun locality at the Beipiao County in Liaoning Province and compromises a partial individual preserving the skull (badly crushed), most of the vertebral column, both arms and other postcrania. Traces of feather integument were extensively found around the neck area.

Beipiaosaurus was a small therizinosaur, with the largest specimen having an estimated length of 2.2 m (7.2 ft). In 2013, Lindsay E. Zanno and Peter Makovicky estimated its body mass at around 27 kg (60 lb), based on the length of its femur. In 2024, Gregory S. Paul estimated Beipiaosaurus' body length at 1.8 m (5.9 ft), and its body mass at around 50 kg (110 lb) More advanced therizinosaurids have four functional toes, but the feet of Beipiaosaurus still have reduced inner toes, showing that the derived therizinosaurid condition may have evolved from a three-toed therizinosauroid ancestor. The head was large relative to other therizinosaurs, with the lower jaw measuring about same length as the femur. The neck appears to be shorter compared to other therizinosaurs. In 2003 the pygostyle, consisting of the fused five last vertebrae of the tail, was described in greater detail, suggesting that the original function of the pygostyle was not linked with pennaceous feathers.

The skull of Beipiaosaurus was fairly large, proportionally, compared to other therizinosaurs. The premaxilla and maxilla are not known, and only one nasal is preserved, though it is not clear which side of the skull it belonged to. The transverse (side-to-side) arching of the nasal observed in other theropods was absent in Beipiaosaurus, though this may be taphonomic. The posterior (rear) half of the right frontal is preserved. The bone overall is inferred to have been subtriangular, as in other therizinosaurs. Unlike other therizinosaurs, the posterior portion is considerably broader than what is preserved of the anterior (front) portion; it is three times the width, as opposed to twice the width. It is flattened, as in Falcarius and Jianchangosaurus, as opposed to the domed shape seen in Erlikosaurus and most coelurosaurs. The parietals are both preserved. They were large and flat, and were separate bones, sutured along the midline; this is contrary to the condition in other therizinosaurs, like Erlikosaurus, where they were fused. The postorbital differs from that of Erlikosaurus in having a proportionally shorter frontal process a longer accessory medial (midline) process, and a more dorsal (higher) rugose area. The posterior half of Beipiaosaurus' skull (the braincase) is represented exclusively by the laterosphenoid and prootic bones.