Nettilling is the largest lake in Nunavut. It is fed by the second largest lake on Baffin Island, Amadjuak Lake; as well as several other smaller lakes and streams. It empties west via the very shallow Koukdjuak River into Foxe Basin. The eastern half has many small islands and the western half is deeper with no islands. The lake is frozen for most of the year. Ringed seals live in the lake, along with three recorded species of fish: the Arctic char, the ninespine, and three-spined stickleback. The tundra around the lake and south to Amadjuak Lake is important for barren-ground caribou feeding and calving.
Nettilling Lake is the eleventh largest in Canada, being one of the largest lakes entirely within Canada.