A new species of Sinemys (Testudines: Cryptodira: Sinemydidae), S. brevispinus n. sp. is described on the basis of three skeletons from the Early Cretaceous Luohandong Formation, Chabu Sumu, Ordos Basin, Inner Mongolia. The new taxon is characterised by an elongate carapace with a pair of distinct but short lateral spines, a sculptured shell surface, closed lateral carapacial fontanelles and an open incisura columellae auris. A nearly complete turtle skeleton from the Early Cretaceous of Hedaochuan, Huanxian, Gansu Province which was previously referred to S. lens is revised and assigned to S. cf. brevispinus. The systematics of Sinemydidae is discussed and the family is restricted to the genus Sinemys. S. brevispinus likely lived in a fluvial environment with the spines acting as a stabiliser to limit the rolling of the carapace, as in S. gamera, but the difference in the shape of the shell suggests that these two species were adapted to different hydrodynamic regimes. The distribution of Sinemys adds to the evidence for biogeographically distinct aquatic communities in eastern and western China during the Early Cretaceous. ? 2012 Senckenberg Gesellschaft f?r Naturforschung and Springer-Verlag Berlin Heidelberg.