Sebago Lake is the deepest and second largest lake in the U.S. state of Maine. The lake is deep at its deepest point, with a mean depth of , covers about in surface area, has a length of and a shoreline length of . The surface is around above sea level, so the deep bottom is below the present sea level. It is located in Cumberland County, Maine and bordered by the towns of Casco, Naples, Raymond, Sebago, Standish and Windham. The seasonally occupied town of Frye Island is on an island in the lake.
The lake is connected to Brandy Pond by the Songo River and eventually to Long Lake in Naples. The name comes from a local Native American tribe. The lake is drained primarily by the Presumpscot River. Prior to construction of a railroad in 1870, Sebago Lake was linked to Portland harbor by the Cumberland and Oxford Canal. The outlet to the river has been dammed since 1875 by the Eel Weir Dam and the Head Dam, owned and operated by the S. D. Warren Paper Mill.
The lake holds roughly 995 billion gallons of water that on average resides 5.1 to 5.4 years in the lake. The direct watershed is about of land plus the of the lake, and the indirect watershed about of land plus about of other bodies of water. As of May 1990, roughly 86% of the watershed was forests and fallow fields, 2.5% in active timber operations, 9.3% in residential, agricultural, and commercial use, and 2.2% used for other purposes. Water inflow is estimated at 544 million gallons per day and outflow at 498 million gallons per day, of which 24 million gallons/day are for the water district.