Lake Östersjön is in south-western Dalsland, Färgelanda Municipality. The lake ’shares’ water with Lake Ellenösjön to the west. Between the two lakes lies Sundsbron and route 172. The Valboån River runs through the lake. Lake Östersjön is somewhat deeper than its neighbour and its water is a little clearer.
Lake Östersjön is surrounded by forested hills and the shore is most often cliffs alternating with gravel. Its character is clearly different when compared with Lake Ellenösjön, leaving a greater sense of wilderness. Though the maximum depth approaches 30 m, there are many shallow coves. Lake Östersjön is known for its pikeperch fishing, but some sports fishermen claim that number of pikeperch caught is smaller, though the fish are larger.