Anteosaurus (meaning "Antaeus lizard") is an extinct genus of large carnivorous dinocephalian synapsid. It lived at the end of the Guadalupian (= Middle Permian) during the Capitanian age, about 265 to 260 million years ago in what is now South Africa. It is mainly known by cranial remains and few postcranial bones. Measuring 5–6 m (16–20 ft) long and weighing about 600 kg (1,300 lb), Anteosaurus was the largest known carnivorous non-mammalian synapsid and the largest terrestrial predator of the Permian period. Occupying the top of the food chain in the Middle Permian, its skull, jaws and teeth show adaptations to capture large prey like the giant titanosuchid and tapinocephalid dinocephalians and large pareiasaurs.

As in many other dinocephalians the cranial bones of Anteosaurus are pachyostosed, but to a lesser extent than in tapinocephalid dinocephalians. In Anteosaurus, pachyostosis mainly occurs in the form of horn-shaped supraorbital protuberances. According to some paleontologists this structure would be implicated in intraspecific agonistic behaviour, including head-pushing, probably to compete with conspecifics over mating rights during the mating season. On the contrary, other scientists believe that these adaptations served to reduce cranial stress on the bones of the skull when biting massive prey.

Young Anteosaurus started their life with fairly narrow and lean skulls, and as they grew up bones of the skull became progressively thickened (a process known as pachyostosis), creating the characteristic robust skull roof of Anteosaurus. The study of its inner ear revealed that Anteosaurus was a largely terrestrial, agile predator with highly advanced senses of vision, balance and coordination. It was also very fast and would have been able to outrun competitors and prey alike thanks to its advanced adaptations. Its body was well-suited to projecting itself forward, both in hunting and evidently in head-butting.

Anteosaurus and all other dinocephalians became extinct about 260 million years ago in a mass extinction at the end of the Capitanian in which the large Bradysaurian pareiasaurs also disappeared. The reasons of this extinction are obscure, although some research have shown a temporal association between the extinction of dinocephalians and an important volcanism event in China (known as the Emeishan Traps).

Some confusion surrounds the etymology of the name Anteosaurus. It is often translated as meaning "before lizard", "previous lizard" or "primitive lizard", from the Latin prefix ante which means "before". The zoologist and paleontologist David Meredith Seares Watson gave no explanation when he named Anteosaurus in 1921. According to Ben Creisler, the prefix does not come from the Latin ante, but would refer to a Giant of the Greek mythology, Antaios, which once Latinized give Antaeus or more rarely Anteus. The type specimen of Anteosaurus is an incomplete skull that Watson had initially classified in the genus Titanosuchus, named after the Titans of the Greek mythology. Once this specimen recognized as belonging to a different genus, the name dedicated to Antaeus established another connection with a giant of Greek mythology.

Named by Watson in 1921, Anteosaurus was longtime classified as a 'Titanosuchian Deinocephalian', and it is only in 1954 that Boonstra separated the Titanosuchians in two families: Jonkeridae (a junior synonym of Titanosuchidae) and Anteosauridae. At about the same time, Efremov erected the family Brithopodidae in which he includes the fragmentary Brithopus and the better known forms Syodon and Titanophoneus. Much later, Hopson and Barghusen argued that Brithopodidae should be discontinued and that the Russian taxa Syodon, Titanophoneus and Doliosauriscus should be placed with Anteosaurus in Anteosauridae. These authors placed also Anteosauridae in the new group Anteosauria for distinguished them of the other major dinocephalian group the Tapinocephalia in which they included the titanosuchids and the tapinocephalids. They also created the taxa Anteosaurinae, containing Anteosaurus and the Russian forms Titanophoneus and Doliosauriscus, and the Anteosaurini containing only the giant forms Anteosaurus and Doliosauriscus. Gilian King retained the incorrectly spelled 'Brithopidae' (including the subfamilies 'Brithopinae' and Anteosaurinae) and placed both Brithopidae and Titanosuchidae (including Titanosuchinae and Tapinocephalinae) in the superfamily Anteosauroidea. Later Ivakhnenko considered Brithopodidae as invalid and united Anteosauridae and Deuterosauridae (only known by the Russian Deuterosaurus) in the superfamily Deuterosauroidea. More recently Kammerer in its systematic revision of the anteosaurs (in which Doliosauriscus become a junior synonym of Titanophoneus) demonstrated that the wastebasket genus Brithopus is a nomen dubium composed both of remains of indeterminate estemmenosuchid-like tapinocephalian and indeterminate anteosaurian, so invalidating the Brithopodidae. He proposed also the first phylogenetic analysis including all anteosaurid taxa. This and other modern phylogenetic analysis of anteosaurs recovers a monophyletic Anteosauridae containing two major clades, Syodontinae and Anteosaurinae. In the Kammerer analysis, the Chinese Sinophoneus is the most basal anteosaurine and the sister-group of an unresolved trichotomy including Titanophoneus potens, T. adamanteus and Anteosaurus.

Below the cladogramm of Kammerer published in 2011 :

In describing the new Brazilian anteosaur Pampaphoneus, Cisneros et al. presented another cladogram confirming the recognition of the clades Anteosaurinae and Syodontinae. In the cladogram of the Fig. 2. of the main paper, which does not include the genus Microsyodon, Titanophoneus adamanteus is recovered as the sister taxon of a clade composed of Titanophoneus potens and Anteosaurus. However, in the four cladograms of the Fig. S1, presented in the Supporting Information of the same article, and including Microsyodon, Anteosaurus is recovered as the sister taxon of both species of Titanophoneus. These four cladograms differ only by the position of Microsyodon.