Polysternon is a genus of turtles in the extinct family Bothremydidae. It was described by Portis in 1882, and contains the species P. provinciale (originally placed in the genus Pleurosternon), which existed during the Cretaceous of what is now France; P. atlanticum, which existed during the Cretaceous of what is now Spain, and a new species, P. isonae, from the Late Maastrichtian of Spain. The species epithet of P. isonae refers to the municipality Isona i Conca Dellà in Catalonia, where the type specimen was discovered.