Lake Nipissing is a lake in the Canadian province of Ontario. It has a surface area of , a mean elevation of above sea level, and is located between the Ottawa River and Georgian Bay. Excluding the Great Lakes, Lake Nipissing is the third-largest lake in Ontario. It is relatively shallow for a large lake, with an average depth of only . The shallowness of the lake makes for many sandbars along the lake's irregular shoreline. The lake has many islands most of which are protected under the Protection of Significant Wetlands scheme, controlled by the Ministry of Natural Resources.