Therocephalia is an extinct group of therapsids (mammals and their close extinct relatives) from the Permian and Triassic periods. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were carnivores. Like other non-mammalian synapsids, therocephalians were once described as "mammal-like reptiles". Therocephalia is the group most closely related to the cynodonts, which gave rise to the mammals. Indeed, it had been proposed that therocephalians themselves may have given rise to the cynodonts, and therefore that therocephalians as recognised are paraphyletic in relation to cynodonts and so not a clade. Conventionally, however, Therocephalia is regarded as the sister clade of Cynodontia, together forming the clade Eutheriodontia.

The close relationship of Therocephalia to Cynodontia takes evidence in a variety of skeletal features. Most notable is that the skull roof is narrowed between two enlarged temporal fenestra, allowing for expansive jaw musculature. At the same time, derived therocephalians also share a number of mammalian traits with cynodonts that evolved convergently, including a secondary palate, loss of the postorbital bar behind the eye and developing multi-cusped cheek teeth for herbivory. Other therocephalians retained simpler teeth for a carnivorous diet, often with large canines and sometimes a reduction or even total loss of the postcanine teeth. Such forms include genera that have even suggested to have possessed a venomous bite (namely Euchambersia), which would make therocephalians the oldest tetrapods known to have evolved this characteristic.

The fossils of therocephalians are most numerous in the Karoo of South Africa, but have also been found in Russia, China, Tanzania, Zambia, and Antarctica. Early therocephalian fossils discovered in Middle Permian deposits of South Africa support a Gondwanan origin for the group, which seems to have spread quickly across the supercontinent Pangaea. Although most therocephalian lineages died out during the Permian–Triassic extinction event, a few representatives of the subgroup Eutherocephalia survived into the ensuing Triassic period. However, only the cynodont-like subgroup Bauriamorpha survived past the Early Triassic and the last therocephalians became extinct by the early Middle Triassic, possibly due to climate change, along with competition with cynodonts and various groups of reptiles — mostly archosaurs and their close relatives, including archosauromorphs and archosauriforms.

Like the Gorgonopsia and many cynodonts, most therocephalians were presumably carnivores. The earlier therocephalians were, in many respects, as primitive as the gorgonopsians, but they did show certain advanced features. There is an enlargement of the temporal opening for broader jaw adductor muscle attachment and a reduction of the phalanges (finger and toe bones) to the mammalian phalangeal formula. The presence of an incipient secondary palate in advanced therocephalians is another feature shared with mammals. The discovery of maxilloturbinal ridges in forms such as the primitive therocephalian Glanosuchus, suggests that at least some therocephalians may have been warm-blooded.

The later therocephalians included the advanced Baurioidea, which carried some theriodont characteristics to a high degree of specialization. For instance, small baurioids and the herbivorous Bauria did not have an ossified postorbital bar separating the orbit from the temporal opening—a condition typical of primitive mammals. These and other advanced features led to the long-held opinion, now rejected, that the ictidosaurs and even some early mammals arose from a baurioid therocephalian stem. Mammalian characteristics such as this seem to have evolved in parallel among a number of different therapsid groups, even within Therocephalia.

Several more specialized lifestyles have been suggested for some therocephalians. Many small forms, like ictidosuchids, have been interpreted as aquatic animals. Evidence for aquatic lifestyles includes scleral rings that may have stabilized the eye under the pressure of water and strongly developed cranial joints, which may have supported the skull when consuming large fish and aquatic invertebrates. One therocephalian, Nothogomphodon, had large sabre-like canine teeth and may have fed on large animals, including other therocephalians. Other therocephalians such as bauriids and nanictidopids have wide teeth with many ridges similar to those of mammals, and may have been herbivores.

Many small therocephalians have small pits on their snouts that probably supported vibrissae (whiskers). In 1994, the Russian paleontologist Leonid Tatarinov proposed that these pits were part of an electroreception system in aquatic therocephalians. However, it is more likely that these pits are enlarged versions of the ones thought to support whiskers, or holes for blood vessels in a fleshy lip. The Late Permian genera Euchambersia, Ichibengops and Megawhaitsia particularly attract the attention of paleontologists because theirs fossil skulls attributed to have some structures which suggests that these animals had organs for distributing venom.

Therocephalia was named by Robert Broom in 1903 as a new order to divide Theriodontia (then essentially containing all known carnivorous Permian and Triassic "mammal-like reptiles") into the "primitive" Permian forms, Therocephalia, and more mammal-like Triassic forms, Cynodontia, based on the anatomy of their palates and the occipital condyle. Broom's Therocephalia was based primarily on Scylacosaurus (effectively the type genus of Therocephalia) and Ictidosuchus, but differs strongly from modern classifications by also including various genera now recognised as gorgonopsians (a group Broom did not recognise as warranting distinction) such as Gorgonops and Aelurosaurus, and even what are now dinocephalians (e.g. Titanosuchus).