The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates(1). Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk(2). Unlike more-derived ichthyosauriforms(3), it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia(4), the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm(5,6) and humid(7) tropical archipelago(8) in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region(9).