Mapusaurus (lit. 'earth lizard') is a genus of giant carcharodontosaurid carnosaurian dinosaur that lived in Argentina during the Cenomanian–Turonian ages of the Late Cretaceous. It is known from a bonebed of between seven and nine specimens, excavated from the strata of the Huincul Formation between 1997 and 2001 as part of the Argentinian-Canadian Dinosaur Project. In 2006, Rodolfo Coria and Philip J. Currie scientifically described Mapusaurus. Only one species of Mapusaurus, M. roseae, has been described, named after the rose-colored rocks in which it was discovered and sponsor Rose Letwin.

Mapusaurus was one of the largest carcharodontosaurids. Based on the biggest specimen known from the bonebed, represented by a left femur, it was originally estimated to have reached a maximum body length of 10.2 metres (33 ft) and a mass of 3 tonnes (6,600 lb). Subsequent works have given maximum size estimates of 10.2–12.6 m (33–41 ft) and 6–8 t (13,000–18,000 lb), respectively. Mapusaurus generally resembled Giganotosaurus, though had a deeper skull, a more rugose maxilla, a rougher surface to its lacrimal bone, differently proportioned neck vertebrae, and various other minor differences. The arms of Mapusaurus were very small, similar in terms of proportional size to those of tyrannosaurids and abelisaurids.

The first fossils of this taxon were discovered in 1995 by members of the Argentinian-Canadian Dinosaur Project in an exposure of the Huincul Formation at Cañadón del Gato, a site 20 kilometres (12 mi) south of Plaza Huincul in Neuquén Province, Argentina. In 1997, crews from the Project began excavating the fossils, which they believed to belong to a single skeleton of a large theropod dinosaur. However, during preparation of the remains it was realized that they came from several individuals of differing sizes and ontogenetic stages. That same year, Argentine paleontologist Rodolfo Coria and Canadian paleontologist Philip Currie, the leaders of the Project, announced the discovery of the theropod at a meeting of the Society of Vertebrate Paleontology, stating that the team had unearthed a single skeleton of a new carcharodontosaurid theropod similar to Giganotosaurus. By that time, an isolated tooth, a surangular, a caudal (tail) vertebra, a manual ungual, an incomplete pelvis, femora, tibiae, a fibula, a metatarsal and several pedal (foot) phalanges had been collected; however, later digs would find more fossils. Excavations of the fossils at Cañadón del Gato lasted from 1997 to 2001, wherein hundreds of fossils from at least seven to nine individuals of Mapusaurus were discovered. These fossils were mentioned in conference abstracts in 2000 and 2001, which noted the possibility of pack behavior or gregariousness in large theropods based on the quantity and age range of the theropod fossils found.

In 2006, Coria and Currie scientifically described the remains, and identified them as belonging to a new genus and species of giant carcharodontosaurid theropod. Based on this material, they named them Mapusaurus roseae. The generic name Mapusaurus derives from the Mapuche word Mapu, meaning "Earth", and the Greek σαῦρος (saûros), meaning "lizard", and thus "Earth lizard". The specific name roseae is named for both the rose-colored rocks, in which the fossils were found and for Rose Letwin, who sponsored the expeditions which recovered these fossils. Coria and Currie designated an isolated right nasal (MCF-PVPH-108.1, Museo Carmen Funes, Paleontología de Vertebrados) as the holotype (name-bearing) specimen of M. roseae. Additionally, Coria and Currie assigned several paratypes to M. roseae, including portions of the skull, limbs, pelvis and vertebrae. In total, Mapusaurus is known from parts of the skull and mandible, teeth, some cervical (neck), dorsal (back) and caudal vertebrae, much of the sacrum and pelvis, some ribs, parts of the scapulocoracoid, much of the hindlimbs, many pedal phalanges, and fragments of the forelimb and manus. Later studies have reidentified some of the elements described by Coria and Currie (2006), such as a carpometacarpus that American paleontologist Matthew Carrano and colleagues (2012) stated is a distal (away from body) humerus and a partial fibula that Canadian paleontologist Phil Bell and Coria (2013) identified as a pathological dorsal rib.

In 2005, a right postorbital of a theropod dinosaur was unearthed by Argentine paleontologist Matias Motta from a section of sandstone strata in Violante Farm in Río Negro Province, Argentina deriving from the lower member of the Huincul Formation. The fossil was then transported to the Museo Provincial "Carlos Ameghino" and cataloged under catalogue number MPCA-Pv 803. In 2016. Motta and colleagues described the postorbital as the holotype of a new genus and species of carcharodontosaurid dinosaur, Taurovenator violantei. Taurovenator went largely unnoticed due to its fragmentary nature, and Coria and colleagues (2019) suggested that Taurovenator is synonymous with Mapusaurus, considering both of the former's autapomorphies (distinguishing traits) were also found in Mapusaurus. Additionally, the authors considered that there was a high likelihood of them being coeval. However, Taurovenator is actually from the lower unit of the Huincul Formation, while Mapusaurus is from the upper unit of the formation, suggesting they could be distinct genera. In 2022, another carcharodontosaurid from the Huincul Formation, Meraxes, was named on the basis of a well-preserved skull and partial skeleton from the same strata as Taurovenator. In their description of Meraxes, the authors stated that Taurovenator lacks sufficient diagnostic characters and may be coeval with Meraxes.

In 2005, an associated skeleton (MPCA-Pv 803) including a partial skull and posterior (back portion) mandible, incomplete cervical (neck vertebrae) series, fragments of dorsal (back) vertebrae, several ribs, two partial forelimbs, a femur, a partial pes, gastralia and a caudal vertebra was unearthed along with the Taurovenator. This specimen was regarded as belonging to an indeterminate carcharodontosaurid in the 2016 description of Taurovenator. In 2024, this specimen was described and assigned to Taurovenator. In an analysis of the strata from which the holotype was discovered, the study noted that the Huincul Formation is separated into two distinct sequences; a lower section of thin, multicolored sandstones and an upper section of thick conglomeratic sediments. Mapusaurus derives from the upper sequence of the formation, whereas Meraxes and Taurovenator are exclusive to the lower rock layers. Meraxes, however, was collected in strata close to the Candeleros-Huincul Formation boundary, whereas Taurovenator's specimens were found over 30 meters above the Candeleros-Huincul Formation limit. It is for these reasons that the three carcharodontosaurids found at Huincul were potentially not coeval, supporting the argument for Taurovenator's validity. Additionally, the holotype preserve features of the Giganotosaurini, further supporting its referral to Taurovenator. A new host of diagnostic traits were found on the bones of MPCA-Pv 803, properly demonstrating its distinctiveness.

In their paper describing Mapusaurus, Coria and Currie estimated that the specimens found in the bonebed measured between 5.5–10.2 m (18–33 ft) in length, with the former being based on a left dentary (MCF PVPH-108.3) and the latter being based on a left femur (MCF-PVPH-108.203). Subsequent maximum size estimates vary from around 10.2–12.6 metres (33–41 ft), and weight estimates range from 6–8 t (13,000–18,000 lb).

The skull of Mapusaurus was deeper and narrower than that of Giganotosaurus, due to the comparative shortness of the maxillae and slenderness of the nasal bones. The nasals were very rugose, as in Carcharodontosaurus, Giganotosaurus, Meraxes and Tameryraptor. The lateral (external, or outer) surface of the maxilla in many carcharodontosaurids (i.e. Carcharodontosaurus, Giganotosaurus, Meraxes and Tameryraptor) had a rough texture, and the same is true of Mapusaurus. Whereas the rugosity of Giganotosaurus's maxilla stopped shortly posterior to (behind) the nasal opening, that of Mapusaurus continued for most of the bone's length. The bar between the antorbital and maxillary fenestrae, the so-called interfenestral strut, was fairly wide in comparison to other carcharodontosaurids. Whereas many derived carnosaurs had several openings in the maxilla anterior to (in front of) the antorbital fenestra, in Mapusaurus, the maxillary fenestra was the only one, and it disappeared with growth. The antorbital fossa was about equal in size to that of Carcharodontosaurus and Giganotosaurus. The orbit, or eye socket, was partly divided into upper and lower sections by projections of the lacrimal and postorbital bones. Like in many derived carcharodontosaurids, such as Meraxes, the lateral postorbital surface bore a robust brow horn. The lacrimals and prefrontal bones were fused, as in many theropods, including Giganotosaurus. The lacrimals of the two genera differed in that Mapusaurus's lacrimal had a rugose dorsal (upper) surface, whereas that of Giganotosaurus bore deep grooves. Mapusaurus's teeth were similar to those of other carcharodontosaurids, being flat, narrow and blade-like and bearing 10–12 denticles per 5 mm (0.20 in), as opposed to 13–15 denticles per 5 mm in Acrocanthosaurus. There were 12 alveoli (tooth sockets) in each maxilla, as opposed to 14 in Carcharodontosaurus.