Tasmaniosaurus ('lizard from Tasmania', although this genus is not a true lizard) is an extinct genus of archosauromorph reptile known from the Knocklofty Formation (Early Triassic) of West Hobart, Tasmania, Australia. The type species is T. triassicus. This genus is notable not only due to being one of the most complete Australian Triassic reptiles known, but also due to being a very close relative of Archosauriformes. Once believed to be a proterosuchid, this taxon is now believed to have been intermediate between advanced non-archosauriform archosauromorphs such as Prolacerta, and basal archosauriforms such as Proterosuchus. Features traditionally used to define Archosauria and later Archosauriformes, such as the presence of an antorbital fenestra and serrated teeth, are now known to have evolved prior to those groups due to their presence in Tasmaniosaurus.

First named as a nomen nudum in 1974, the genus received a formal description by paleontologists Charles Lewis Camp and Maxwell Banks in 1978. These descriptions considered it a proterosuchid archosaur. A redescription by British paleontologist Tony Thulborn in 1986 agreed with this interpretation. Since then, cladistic work has redefined the term "archosaur" to only include Avemetatarsalia (a lineage including pterosaurs and dinosaurs, such as modern birds) and Pseudosuchia (a lineage including modern crocodylians and their extinct relatives such as aetosaurs and raisuchids). As proterosuchids evolved prior to the split between these two groups, they are not considered archosaurs using this definition. In lieu of this revelation, the clade Archosauriformes is now used to encompass proterosuchids and archosaurs (as well as several other families) under one group. Archosauriformes is itself a component of Archosauromorpha, a broader clade which refers to all animals more closely related to archosaurs than to lepidosaurs, the other main group of reptiles including lizards, snakes and tuataras.

During this transition, Tasmaniosaurus remained ignored. This was rectified when the genus received a thorough redescription by Martín Ezcurra in 2014. In 2016, Ezcurra also included the genus in his comprehensive analysis of Archosauromorphs, which indicated that proterosuchidae (as it was usually defined) was an invalid polyphyletic grouping. This analysis included a phylogenetic analysis which incorporated Tasmaniosaurus and found that it was not in fact a proterosuchid. Rather, it was found to be the sister taxon of Archosauriformes, meaning that it was the closest known relative of members of that clade without technically being part of it (as it was not closer to either proterosuchids or other archosauriforms).

Tasmaniosaurus is known from a single partial skeleton, UTGD (University of Tasmania School of Earth Sciences) 54655. This holotype specimen consists of various skull fragments, vertebrae, ribs, an interclavicle and bones of the back legs. The specimen as a whole is jumbled and missing many elements, and some of the bones preserved within it have not been identified with absolute certainty. Even so, it is considered one of the most complete skeletons of any Triassic reptile unearthed in Australia. A few other bone fragments collected around Tasmania have been occasionally referred to this genus, but they are currently considered indeterminate and lost.

The premaxilla (a tooth-bearing bone forming the snout tip) was initially mistaken to be very short due to crushing. However, it was later found to be proportionally similar to that of most archosauriforms. It is rounded from the front and possesses a long and tall 'maxillary process' (rear extension). By comparing the orientation of this process with the tooth row, the snout tip was determined to be only slightly projected downwards, in contrast to the drastically hooked snout of putative proterosuchids. Although only a few teeth are preserved in the right premaxilla, a count of the tooth sockets helps estimates that 6 or 7 teeth were present in each premaxilla during life.

The maxilla (a tooth-bearing bone on the side of the snout) has a long tooth row and a tapering rear tip. The front tip also forms a tapering 'anterior process' which smoothly transitions into a triangular and upward-projecting 'ascending process'. This contrasts with proterosuchids, which have a less abruptly tapering anterior process, and erythrosuchids, which have a pillar-like ascending process. The shape of the upper edge of the maxilla indicates that Tasmaniosaurus had an antorbital fenestra, a hole in the side of the snout which seemingly characterizes archosauriforms. The presence of an antorbital fenestra supports the very close relation between Tasmaniosaurus and archosauriforms. As the skull bones are all preserved lying face down, it is difficult to assess whether an antorbital fossa (a depression which rings around the antorbital fenestra) was also present. The left maxilla preserves 14 teeth while the right preserves nine. An estimated 21 teeth were present in each maxilla during life. The lacrimal bone (in front of the orbit, or eye hole) is L-shaped and particularly similar to that of Proterosuchus. On the medial (inside) face, a large tuberosity (bony bump) is present where the forward and downward extensions meet. A partial pterygoid bone (a tooth-bearing part of the roof of the mouth) is preserved in the specimen, and is almost identical to that of Proterosuchus and Prolacerta. It preserves six or seven teeth, and likely represents the front part of the pterygoid.

Several bones of the skull roof were also preserved connected to each other in the holotype. Camp & Banks considered these to be frontals, parietals, an interparietal and postfrontals, all bones of the rear of the skull. Thulborn instead interpreted them as frontals, nasals and postorbitals, on the upper side of the snout. Most recently, Ezcurra discussed both of these interpretations and concluded that Camp & Banks were correct in their identification of the bones. The frontals are long and unfused, and possess thin "finger-like" extensions which would have connected to the nasals. Each postfrontal, which formed the upper rear edge of the orbit, is similar to that of Archosaurus but the extent of its contact with the other bones is unclear. The parietals are unfused and have wide and concave outer edges, forming the inner edge of the upper temporal fenestrae (a pair of large holes on each side of the back of the head). The back of each parietal has a long bony rod which extends backwards and curves outwards (a posterolateral process), forming an angle of about 20 degrees with the midline of the skull. A large crescent shape interparietal lies at the back of the skull roof, between the posterolateral processes of the parietals, similar to proterosuchids. Two smaller bone fragments were also found near the skull roof and may have been a supraoccipital and epipterygoid (both bones of the braincase), although such an assignment is uncertain.

The dentaries (the main tooth-bearing bones of the lower jaw) are long, thin and straight, similar to those of Prolacerta and Protorosaurus but contrasting with the robust and/or upwards-curving jaws of most basal archosauriforms. In fact, the tooth row at the very tip of the jaw slightly curves downward, forcing the first few teeth to project a bit forwards as well as upwards. The rear edge of each dentary has two tapering bony extensions, a short (but partially broken) 'posterodorsal process' on top and a much more prominent 'central posterior process' on the bottom. The dentaries are long enough that the front tip extends almost as far forward as the snout tip while the tooth row would extend almost as far back as the tooth row of the maxilla, both features unlike Prolacerta and Proterosuchus. Only five teeth are preserved in the left dentary, but more than 22 were likely present in life. A thick left splenial (a bone of the inside and lower edge of the lower jaw), similar to that of Proterosuchus, is also preserved.