Abstract: A new eosauropterygian, Diandongosaurus acutidentatus gen. et sp. nov., is described based on a skeleton from the Upper Member of Guanling Formation (Middle Anisian) of Luoping County, Yunnan Province, southwestern China. This new species displays a combination of traits seen in nothosauroids (Simosaurus and nothosaurians) and pachypleurosaurs (Dactylosaurus, Anarosaurus, Serpianosaurus, and Neusticosaurus). It has an unconstricted snout, preorbital region longer than the postorbital region, and a supratemporal fenestra smaller than the orbit, as in the pachypleurosaurs. On the other hand, it bears enlarged and procumbent teeth in the premaxilla and the anterior dentary, and one or two fang-like maxillary teeth, as in the nothosaurs. In this new species, the frontals and the parietals are fused, the posterolateral process of the frontal extends posteriorly over the anterior margin of the supratemporal fenestra, the postorbital is excluded from the infratemporal fenestra by the jugal-squamosal contact, the quadratojugal is present, the clavicle has an anterolateral projection, and the three sacral and anterior caudal ribs are distally pinched off but not constricted. Unlike in most of other stem eosauropterygians, the prefrontal and the postfrontal meets each other along the dorsal margin of the orbit, the descending ramus of the squamosal reaches the articular condyle of the quadrate, caudal ribs 3 to 8 are elongate and longer than sacral ribs, and the ungual phalanxes of the pes are extremely dorsoventrally expanded. Our phylogenetic analysis suggests that Diandongosaurusis neither a pachypleurosaur nor a nothosauroid; it might be the sister group of the clade consisting of Wumengosaurus, the nothosauroids and those taxa traditionally considered as pachypleurosaurs.