The diversification of marine reptiles played a major part in the Triassic Revolution and represented the first large-scale return of tetrapods to an aquatic environment. Recently, a new marine archosauromorph clade, Dinocephalosauridae, was recognised, considerably increasing the known diversity of Triassic marine reptiles, particularly among the generally terrestrial Archosauromorpha. Its best-known member, Dinocephalosaurus orientalis, superficially resembles plesiosaurs, possessing a hyperelongate neck composed of many cervical vertebrae, an elongate torso, and flipper-like limbs. The osteology of Dinocephalosaurus is virtually completely described based on newly discovered specimens. Up to six metres long, it is characterised by its long tail and even longer neck. The appendicular skeleton exhibits a high degree of skeletal paedomorphosis recalling that of many sauropterygians, but the skull and neck are completely inconsistent with sauropterygian affinities. Its cranial morphology, including the presence of narial fossae, is very similar to that seen in another long-necked archosauromorph, Tanystropheus hydroides, which largely represents a convergence related to an aquatic piscivorous lifestyle. Chinese discoveries such as Dinocephalosaurus merit a re-evaluation of historical European collections. Based on such a revision, we also redescribe Trachelosaurus fischeri, known from a single, disarticulated specimen collected in the 1800s from the Solling Formation (Buntsandstein) of Bernburg, Germany. It possesses short, bifurcating cervical ribs, which are unique among archosauromorphs. Trachelosaurus is confidently recognised as the first European dinocephalosaurid based on a wide range of character states, including its highly presacral vertebral count, wide dorsal transverse processes, holocephalous dorsal ribs, an ilium lacking a preacetabular process, and a rod-like femur.