Jianchangosaurus ("lizard from Jianchang County") is a genus of therizinosaurian dinosaur that lived approximately 126 million years ago during the early part of the Cretaceous Period from the Yixian Formation in what is now China.

The type specimen, a juvenile, was discovered in Jianchang County, in the western part of Liaoning Province.

In spite of its basal position, Jianchangosaurus already bore many of the hallmarks of therizinosaur anatomy, such as a beaked premaxilla, and the shape of the pubic boot.

The holotype of Jianchangosaurus (41HIII-0308A) is the nearly complete skeleton of a single juvenile, missing only the distal (far) portion of the tail, and some minor elements.

In 2013, Hanyong Pu, Yoshitsugu Kobayashi, Junchang Lu, Li Xu, Yanhua Wu, Huali Chang, Jiming Zhang, and Songhai Jia published a paper describing 41HIII-0308A.

The genus name is derived from the county where the specimen was found, and the Greek word "sauros" (σαυρος), meaning "lizard".

The specific name yixianensis, refers to the Yixian Formation where the specimen was found, and the Latin suffix "-ensis" meaning "originating in".

[1] Gregory S. Paul estimated the weight of the holotype specimen around 20 kg (44 lb), though expressed uncertainty about its total body length, due to the fact that tail tip is unpreserved.

[2] In 2024, he suggested a higher body mass of 26 kg (57 lb)[3] The skull of Jianchangosaurus' holotype is fairly well-preserved, lacking only a few elements.

The prefrontals are triangular, with a V-shaped anterodorsal region (at the top and towards the front) which contacts the lacrimal.

Jianchangosaurus' dentary was triangular in lateral view, with a downturned symphyseal region, as in other therizinosaurs (with the exception of Falcarius).

A similar dental morphology is seen in ornithopods and ceratopsians, and likely maximised biting stress during occlusion, allowing Jianchangosaurus to more effectively slice plant fibres.

The neural arches of the middle and posterior cervicals were X-shaped in dorsal view, like other therizinosaurs, and like oviraptorosaurs.

The anterior dorsal vertebrae lacked prominent hypapophyses; finer details are difficult to discern, as in the holotype they are obscured by the ribs.

At least sixteen gastralia, bones which would have supported the abdominal organs and served as muscle attachment sites, were present.

[1] The scapula and coracoid of Jianchangosaurus were unfused, like in other basal therizinosaurs, though this may be due to its nature as a juvenile.

The tibia is around half as long again as the femur, the highest ratio seen in therizinosaurs;[1] such an adaptation strongly correlates to cursorial habits in dinosaurs.

[1] Left cladogram is based on the phylogenetic analysis conducted by Pu et al. 2013, showing the relationships of Jianchangosaurus as a very primitive therizinosaur.