Vostervatnet is a lake in Strand Municipality in Rogaland county, Norway. The 2.62-square-kilometre (1.01 sq mi) lake lies in the northwestern part of the municipality, just south of the Fognafjorden. The village of Tau lies about 3.5 kilometres (2.2 mi) southwest of the lake. The main outflow of the lake is the river Fiskåna, which flows north from the northeastern corner of the lake, down the hill to the village of Fiskå on the shore of the fjord.