Hupehsuchus, an aquatic reptile from the Triassic of China, represents a previously unrecognized diapsid order, the Hupehsuchia. Hupehsuchus exhibits a unique combination of skeletal features including very long, completely toothless jaws, bipartite neural spines, a complex pattern of dermal armour above the vertebral column, and laterally compressed, spindle-shaped body form. The pattern of vertebral articulation focuses the greatest amplitude of lateral undulation in the posterior trunk and caudal region. Many derived features of Hupehsuchus are also observed in other groups of Mesozoic diapsid reptiles, but no specific sister-group relationship can be established. The difficulty in determining the relationship of the Hupehsuchia may be attributed to the limited knowledge of the fossil record of diapsid reptiles in the late Permian and early Triassic, as well as the great amount of convergence exhibited by secondarily aquatic reptiles. The principle of parsimony cannot be used directly to identify homologous characters if most of the derived characters are convergent.