An almost complete Nothosaurus specimen is described in particular from the Upper Member of the Guanling Formation (Pelsonian, middle Anisian, Middle Triassic) of the Xinmin District, Panxian County, Guizhou Province, Southwestern China. The specimen provides new information on the postcranial osteology of the species. The diagnosed features of the species are revised by the following characters: four premaxillary fangs followed by one small conical premaxillary tooth and five to six small maxillary conical teeth followed by paired maxillary fangs, no posterior process in the interclavicle, four carpals, four sacral vertebrae and hyperphalangy in both manus and pes. In some aspects of its postcranial morphology, GMPKU-P-3014 appears to approach Lariosaurus more closely than Nothosaurus, such as no posterior process in the interclavicle, four carpals, four sacral vertebrae and hyperphalangy in both manus and pes, which provides the important characters to reevaluate the phylogenetic relationships of Nothosaurus and Lariosaurus within the Sauropterygia. GMPKU-P-3014, GMPKU-P-1080 and IVPP V 14294 are different from all other nominal species of Nothosaurus by their size, dentition and the skull future pattern. In addition they have the most similar shape and important proportions of the skull, as well as the same location and stratigraphic level. Thus, they are the same species and indicates that N. rostellatus is a junior synonym of N. yangjuanensis and should be abandoned.