Cyamodontoidea is an armoured lineage of Placodontia, the basal-most clade of Sauropterygia. It was unknown in China until Sinocyamodus xinpuensis was reported in 2000, on the basis of a subadult specimen. Here we describe the first adult specimen of this taxon. This nearly complete specimen was determined to be S. xinpuensis by the stout triangular skull with a short snout, the nasal–nasal contact, a carapace with three types of osteoderms developing rows in meridional ridges and vaulted zones on both sides and the phalangeal formula of 2-3-4-4-2. In this study, the diagnosis of S. xinpuensis is revised based on the new specimen, and the morphological interpretation, especially of the postcranial anatomy, is given. With new information from the adult specimen, our phylogenetic analysis supports the sister-group relationship of Sinocyamodus to Cyamodus. The differences between this new adult specimen and the subadult type specimen reveal ontogenetic variations in morphology, such as the growth of the posterior portion of the skull, the number of osteoderms and the configuration of the carapace. ADDITIONAL KEYWORDS: comparative anatomy – ontogeny – osteology – phylogeny – postcranial skeleton – Reptilia –Triassic.