Tewkensuchus ("forehead crocodile") is an extinct genus of large-bodied sebecoid notosuchian from the Paleocene of Argentina. The genus was described on the basis of fragmentary skull remains alongside a few vertebrae and finger bones collected from the Salamanca Formation. Though likely a terrestrial predator akin to genera like Sebecus, Tewkensuchus much more closely resembles European forms such as Iberosuchus, Bergisuchus and Dentaneosuchus. These European genera form a clade with Tewkensuchus, which appears to be the sister group to the Sebecidae of South America, with both groups being placed in the clade Sebecoidea. Gonzalo Gabriel Bravo and colleagues furthermore note the great size of the animal, which based on its estimated skull length may have weighed around 300 kg (660 lb), not only making it larger than the largest definitive sebecosuchians of the Cretaceous but also putting it far above the maximum weight generally assumed for terrestrial species that survive the Cretaceous–Paleogene extinction event. Possible explanations for this unusually large size might be that the ancestors of Tewkensuchus and other sebecoids could have been more semi-aquatic than their descendents or that they were terrestrial and simply grew rapidly following the extinction of the dinosaurs.

The fossil remains of Tewkensuchus were discovered during field campaigns that took place in 2008 and 2022 in the Punta Peligro area of Argentina's Chubut Province. The material stems from sediments assigned to the Salamanca Formation, which dates to the Danian stage of the Paleocene, making the fossils approximately 63 million years old.

The name Tewkensuchus is derived from the Tehuelche word "t'ewk'en" or "t'ewq'en", which translates to "forehead", a word specifically chosen to reference the unique anatomy of the animal's frontal bone. As often in fossil crocodyliforms, the second part of the name is composed of the word "suchus", derived from the Ancient Greek word "soukhos", itself taken from the Egyptian crocodile-headed god Sobek, who is also namesake of the clades Sebecoidea and Sebecosuchia.

Tewkensuchus is only known from fragmentary remains, primarily parts of the skull roof and a few postcranial remains. The skull remains are represented primarily by a frontal bone and the connected postorbitals and palpebrals as well as part of a prefrontal. As in all mesoeucrocodylians, the frontal is a single fused element that occupies the space between the eyesockets and forms the front of the skull table. The frontal is dominated by two broad depressions located on either side of a sagittal crest, which is similar to what can be seen in certain baurusuchids and the European sebecosuchians Dentaneosuchus and Iberosuchus. The sagittal crest is described as broad and high, which actually serves to set Tewkensuchus apart from the related sebecids of South America, who's sagittal crests are generally described as narrow and ridge-like, though their height may vary between species. The overall ornamentation of the frontal's surface is also regarded as being diagnostic for Tewkensuchus, beginning as pronounced subparallel grooves at the anterior (front) end of the bone and transitioning to shallower grooves with less clear orientation and length further towards the back.

While relatively narrow in the front, the frontal bone rapidly expands towards its posterior section where it contacts the postorbitals to either side, effectively creating a 90° angle at the inner corner of the eyesockets. Prior to contacting the postorbital, the frontal bears a prominent semicircular shelf that is recessed from the rest of the bones surface and continues onto the adjacent part of the postorbital. This depression has been noted to resemble a similar structure also present in the postorbitals of other notosuchians such as sphagesaurids, peirosaurids and uruguaysuchids. However, unlike in these groups and like in the European Iberosuchus the depression is not overlapped by the palpebral. Another notable feature visible at the anterior edge is the presence of a high vertical facet that, due to its concave morphology, serves to articulate with the slightly convex posterior palpebral bone. Towards the lower middle of this facet, the postorbital bears a narrow socket, a counterpart to the peg-like structure present on the corresponding palpebral. Such a peg and socket articulation is unique to Tewkensuchus among crocodyliforms in their entirety. Yet another feature of the postorbital noted by the team naming Tewkensuchus is their overall orientation relative to the rest of the skull table. Typically in crocodyliforms, the skull table is a flattened element with an overall even horizontal surface. However, in Tewkensuchus the postorbitals display a clear incline relative to the surface of the frontal, meaning that the outer edges of the postorbitals are positioned higher than the rest of the known skull table.

Two palpebrals would have been located above either eye of Tewkensuchus, one subquadrangular anterior palpebral (although its true shape remains unknown due to the damage suffered by the holotype) and one more triangular posterior palpebral that would articulate with the postorbital via the sock and peg articulation.

Tooth bearing fragments of the snout and an isolated tooth are known from the holotype of Tewkensuchus, showing that its dentition seemingly differed noticeably from that of sebecids. The tooth crown is described as conical and its inner (labial) and outer (lingual) surfaces as convex, with a sharp carina formed by the medial and distal edges of the crown. This means that though the teeth possessed a sharp cutting edge, they were not ziphodont as they lack the serrations and the sideways compression seen in sebecids like Sebecus.

Some postcranial remains are also known from Tewkensuchus, though they are incomplete and badly damaged. The holotype included six neck vertebrae, 14 phalanges including 6 representing the very tip of their respective finger or toe and finally eight tail vertebrae.