Mississippi Lake is a lake in Lanark County in Ontario, Canada. Ontario's Mississippi River flows northeast and north through the lake. Several small creeks including Cranberry Creek, McCrearys Creek, and McGibbon Creek drain into the lake from adjoining forest and agricultural land. The lake is distinctive for having one side (to the north) that is part of the Canadian shield, while the other is mostly limestone. The lake is a remnant of the old Champlain Sea, which flooded eastern Ontario at the end of last ice age.