Seljordsvatnet (lit. 'Lake Seljord') is a lake in Seljord Municipality in Telemark county, Norway. A small part of the very far eastern edge of the lake crosses over into Midt-Telemark Municipality. The 16.49-square-kilometre (6.37 sq mi) lake is part of the Skien watershed. The lake's main inflow is the river Vallaråi and the lake drains through the river Bøelva. According to local folklore, Selma the sea serpent (Seljordsormen) lives in the lake.

The village of Seljord is located on the northwestern end of the lake. The European route E134 highway and the Norwegian National Road 36 are located on the shoreline of the lake on the northwestern and northern sides of the lake.