Lake Femsjøen is the southernmost of the six lakes in the Haldenvassdraget lake system. Located in Halden Municipality it empties the system in a steep fall into the Tista River and from there runs into the sea through Haldenfjorden and Svinesund Sound. This lake is deep and wide, as opposed to its more northern sisters. The lake surface is broken up by its many islands and sounds, creating excellent fishing places.
Lake Femsjøen is largely lacking in nutrients and thus has clear water. The shoreline is mostly composed of deciduous and coniferous trees, though there are some cultivated fields in the north and southwest. This is where there are shallows, as well as around the islands in the lake. There are some cabins around the lake and at the southern tip at Tistedalen there are villas and multifamily housing.