Lugnsjön Nature Reserve covers 36 hectares at Lugnet, three kilometres east of Söderhamn. Its shallow, nutrient-rich lake is visited for education and recreation, and is a habitat for birdlife. Walking and cycling paths run through the southern part; trails and a bird tower are also in the reserve.

Birds nesting at the lake include great crested grebe, black-necked grebe, tufted duck and pochard. Moorhen and water rail are regular visitors. The lake also holds insects, crustaceans, worms and spiders, which feed fish and birds. Visitors can bring a net and a shallow plastic tray, or a water viewer, to inspect the small animals.

The southern shore has woodland with alder and plants including sårläka, slokstarr and hässlebrodd. The reserve says the woodland contains most of the deciduous tree species found in northern Sweden. Blue anemones flower on the slope in spring; sårläka has its northernmost growing site in Sweden here.

Lugnsjön was lowered by about one metre in the early 1900s to gain land. As the lake gradually became overgrown, volunteers, with municipal support, saved it in the early 1990s.