Trachyteuthis (meaning "rough squid") is an extinct genus of octobrachian cephalopod comprising nine species with an almost global distribution, living from the Callovian to the Cenomanian age of the Mesozoic. While similar in appearance to modern squid or cuttlefish (Decapodiformes), this taxon is actually more closely related to vampire squid.

The first specimen of Trachyteuthis from the Solnhofen Lithographic Limestone was illustrated in 1773 by Georg Wolfgang Knorr, who erroneously identified it as fish remains. The first formal description of what would later become Trachyteuthis was conducted by Dr. Eduard Rüppel in 1829. Rüppel described his specimen as Sepia hastiformis due to the superficial similarity the gladius of Trachyteuthis bears to that of the extant cuttlefish genus Sepia. Following Rüppell’s original description, Georg Graf zu Münster recognised a further 7 species from the Solnhofen Limestones in 1837 but was unable to give either descriptions or illustrations of these species after falling sick, leaving Alcide d'Orbigny to publish them in a series of publications based on notes given to him by Münster. The whole of Münster's unfinished manuscript was published after his death in 1846, within "Beiträge zur Petrefaktenkunde". Meanwhile, Férrussac and d'Orbigny revised Münster's ideas, mentioning that the differences between Münster's proposed species are most likely the consequences of ontogeny or preservational artefacts.

In 1846 Christian Erich Hermann von Meyer pointed out the morphological differences between the gladii found in Solnhofen and those of modern cuttlefish, choosing to erect the genus Trachyteuthis. However, he disregarded previous publications, choosing to name two new species T. oblonga and T. ensiformis, disregarding S. hastiformis. In the meantime the first specimens to later become a part of Trachyteuthis were described from outside the Solnhofen Limestone in 1855 with Richard Owen's description of material from the Kimmeridge Clay, which he named Coccoteuthis latipinnis. In the following decades various species of Trachyteuthis would be described from all across the world; however, the exact taxonomic affinity of the genus has been under debate. Early hypotheses interpreted Trachyteuthis and related taxa as "fossil teuthids" or as members of Sepiidae. Later on Bandel and Leich introduced an alternative idea which has now become generally accepted: that these animals are members of the Vampyromorpha, pointing out that all of them have 8 arms lacking tentacles.

Dozens of well-preserved specimens of T. hastiformis and T. nusplingensis, as well as the related Glyphiteuthis libanotica from the Sannine Formation of Lebanon, allow for a detailed reconstruction of not just the gladius morphology of this genus but also of its soft tissue structures.

The gladius, the chitinous pen in the dorsal mantle, is a sturdy but flexible structure found today in Vampyromorpha, Oegopsida, Myopsida, and Sepiolida. It is commonly regarded as originating from the proostracum of belemnoids. Trachyteuthis itself has a teudopseid-type gladius; the anterior extremity is rounded, and in contrast to the gladii of loligosepiids, the hyperbolar zones and lateral fields are shortened. Most characteristic of Trachyteuthidae as a whole are sepiid-like granules (tubercles) on the dorsal surface of the gladius.

In a study on 50 Trachyteuthis specimens from the Solnhofen and Nusplingen Limestones, Fuchs (2007) describes three different morphotypes in respect to the median field of the gladius which represent the 3 different species found in these deposits. The non-planar type, which is found in the type species T. hastiformis, shows a distinct spindle-shaped elevation, surrounded by a depression, along the longitudinal axis of the median field. In comparison, the gladii of T. nusplingensis exhibit a planar type with no visible depression or elevation on the dorsal side of the gladius. Lastly, T. teudopsiformis shows a prominent, well-developed keel on the dorsal surface of the gladius, similar to the Early Jurassic Teudopsis. More distinct morphologies are found in the other species of Trachyteuthis.

The beak of Trachyteuthis is known by remains of T. nusplingensis from the Nusplingen Limestone; two specimens preserve a lower and upper jaw in association with the gladii. Further isolated beaks are known from the same deposit. In its flattened state the lower beak displays a distinct U-shaped outline of the outer lamella with a hood consisting of two dorsolateral lappets interconnected by a narrow band of carbon coating. In most specimens a subtle three-dimensional relief is visible; this relief is smoothly rounded and very shallow, implying that the beak tip is rounded and not pointed, similar to the beak of the extant Octopus vulgaris.

The mantle musculature of gladius-bearing cephalopods like Trachyteuthis usually corresponds to the shell length. A muscular mantle can be recognised in the fossil by the striated appearance created by the alternation of circular and radial muscles, which in modern cephalopods are continuous around the mantle; however, in Trachyteuthis, they extend for only a short distance, which might be a more basal state than modern taxa. The mantle of Trachyteuthis is exclusively attached to the exterior margin of the gladius. A fusion of the head and mantle as seen in modern octopods and some other Jurassic octobrachians cannot be identified in Trachyteuthis, implying a more distinct head akin to modern decapodiformes.