Lake Viljandi (Estonian: Viljandi järv; 1.55 km²) is a lake in Viljandi County, Estonia. The lake lies in the deep primeval valley of Viljandi, depth 11 m, length 4.6 km, width 450 m. The Uueveski and Valuoja streams and a number of springs flow into the lake. In the south-western part of the lake the Raudna River flows out. The main fish in the lake are bream, roach, perch and pike.