The Valdayka is a river in Valdaysky District of Novgorod Oblast and in Bologovsky District of Tver Oblast in Russia. It is a tributary of Lake Piros and belongs to the basin of the Msta River. It is 50 kilometres (31 mi) long, and the area of its basin 783 square kilometres (302 sq mi).The Valdayka flows over the Valdai Hills and is a part of the waterway from Lake Valdayskoye to Lake Ilmen. The source of the Valdayka is in Lake Uzhin, close to the town of Valday.
The river flows north-east to Lake Piros. It runs through a number of lakes 3 kilometres (1.9 mi)–4 kilometres (2.5 mi) long and 300 metres (980 ft)–400 metres (1,300 ft) wide, in particular, Lake Zakidovskoye and Lake Plotichno. The Moscow-Saint Petersburg Railway crosses the Valdayka close to the village of Lykoshino. Since the Berezayka River flows through the Lake Piros, the Valdayka is considered its tributary.