The Syas River (Russian: Сясь) is a river in Lyubytinsky District of Novgorod Oblast and Tikhvinsky and Volkhovsky Districts of Leningrad Oblast, Russia. The Syas River flows from Valdai Hills north into Lake Ladoga. A town of Syasstroy is located at its mouth. It is 260 kilometres (160 mi) long, and the area of its basin 7,330 square kilometres (2,830 sq mi). The largest tributary of the Syas is the Tikhvinka River (right).