The Kasplya is a river in Smolensky, Demidovsky, and Rudnyansky Districts of Smolensk Oblast of Russia and in Vitebsk Region of northern Belarus. It is a major left-bank tributary of the Daugava. Of its total length of 136 kilometres (85 mi), the first 116 kilometres (72 mi) are in Russia, and the rest in Belarus. It joins the Daugava in the urban-type settlement of Surazh. The town of Demidov is situated on the Kasplya.