Zuolong (/ˌtzwoːlʊŋ/) is an extinct genus of tetanuran theropod from the Late Jurassic period of China. The type and only species is Z. salleei. The generic name of Zuolong is in honor of General Zuo Zōngtáng (also known as "General Tso") with the Chinese word "long" which means dragon. The specific epithet "salleei" is in honor of Hilmar Sallee, who funded the expedition which led to the specimen's discovery.

Zuolong was discovered in the upper part of the Wucaiwan member of the Shishugou Formation in Xinjiang, China. 40Ar/39Ar dating of volcanic feldspar at this locality places it at the span between the Callovian and Oxfordian boundary, and Zuolong was discovered in the upper part of this unit, which is interpreted as being Oxfordian in age. The specimen was discovered in 2001 by the Sino-American field expedition, but it was not described until 2010 when Jonah Choiniere, James Clark, Catherine Forester, and Xu Xing published a full analysis of the bones.

Choiniere and colleagues noted that, at the time of its description, Zuolong was one of the oldest coelurosaurs known to science, but that the implications of its discovery cannot be fully understood until more fossil material is discovered. The Middle Jurassic preserves very few coelurosaurs, and the ones which are known are almost all from China, with the exception of Proceratosaurus and Kileskus.

The holotype of Zuolong, given the designation IVPP V15912, consists of a partially complete skull and numerous post-cranial elements. The skull preserves a maxilla, a premaxilla, one of the quadrate bones, both quadratojugals, a squamosal bone, both ectopterygoids, a pterygoid bone, a lacrimal bone, a postorbital bone, a partial frontal and parietal, as well as three of the teeth from the lower jaw. Other elements of the skeleton which have been preserved include five cervical vertebrae, four dorsal vertebrae, five sacral vertebrae, eight caudal vertebrae, a humerus, the radius and ulna from the left arm, one of the hand claws, the left ilium, both pubic bones, both femora, a tibia, part of a fibula, three metatarsals from the right foot, three toes, and a single toe claw.

Zuolong was not a large theropod. Choiniere and colleagues used two regression analyses based on the work of P. Christiansen and R.A. Fariña as well as François Therrien, and Donald M. Henderson to estimate the body mass of Zuolong and they calculated a range of between 16–50 kilograms (35–110 lb) based on the length of the femur and the size of the skull. This would make it about half the estimated size of its contemporary, Guanlong. Later, Gregory S. Paul suggested that the holotype is a juvenile and estimates a total adult length of 3 metres (9.8 ft) and a mass of 50 kilograms (110 lb). Other authors have suggested a larger adult size, giving a total length of 3.35 metres (11.0 ft) meters and a mass of 43 kilograms (95 lb) kilograms. The holotype is also considered by Thomas R. Holtz Jr. to almost certainly be from a juvenile theropod.

Choiniere and colleagues provide the following traits as autapomorphies for the skull: a slit-like depression on the surface of the quadrate bone, a square-shaped premaxillary body, a triangular tapering at the anterior of the maxilla, a relatively shallow antorbital fossa, frontal and jugal processes of the postorbital bone which contact at a right-angle, a postorbital bone with no anterior process, a ventral anterior process of the lacrimal bone. They also describe several autapomorphies of the post-cranial skeleton including: a centrum of the fifth sacral vertebra with an obliquely angled posterior articulation, a large fovea capitis, a large distal condyle of the third metatarsal, a short post-acetabular wing of the ilium, and a lack of a pubic tubercle, a straight ulna and radius, a ridge on the head of the tibia, a lack of paired lateral foramina on the vertebrae, a lack of lateral fossae on the vertebral centra, a straight humeral and femoral shaft, and a high and rounded ilium.

Several parts of the skull of the holotype are preserved completely, albeit with very poor preservation quality. This makes some aspects of the skull anatomy difficult to determine, but enough is known that the authors who described it noted several distinct features. There are very few primitive coelurosaurs known from complete remains, however the authors are able to draw numerous distinctions between Zuolong and other Late Jurassic small theropods such as Guanlong, Coelurus, and Tanycolagreus. The skull is overall triangular-shaped, with a significant tapering towards the end of the snout. It has very large orbits which face laterally and a pronounced anterior process of the lacrimal, which gives the appearance of a small crest above the eyes, a trait very common among theropods.

The preserved alveoli of the tooth positions are relatively well-preserved, which led Choiniere and colleagues to estimate that in life, Zuolong likely had a total of four premaxillary and twelve maxillary teeth. Of the teeth which are preserved, one is likely a premaxillary tooth, because it is said to be much smaller than the other preserved teeth. It is d-shaped in cross-section, which is the condition seen in tyrannosauroids, although they are not quite as convex as they are in those taxa. The other teeth which are preserved were badly damaged by the fossilization process, but they do appear to have some serrations. In most other respects, they resemble the teeth of most other theropods; they are long and recurved with cylindrical roots. This is emblematic of a common trend in the skull anatomy of Zuolong which the authors note. It shares numerous skull characteristics with derived coelurosaurs, but also with more basally-branching theropods like carcharodontosaurs and megalosauroids.