Lake Winnebago (Menominee: Wenepekōw Nepēhsæh, Ojibwe: Wiinibiigoo-zaaga'igan, Oneida: kanyataláheleˀ) is a shallow freshwater lake in the north central United States, located in east central Wisconsin. At 137,700 acres, it is the largest lake entirely within the state, covering an area of about 30 miles by 10 miles with 88 miles of shoreline, an average depth of 15.5 feet, and a maximum depth of 21 feet. It has many shallow reefs along the west shore and a drop-off type shoreline on the east. There are several islands along the west shore.

The lake has two primary tributaries, the Wolf and Fox Rivers, which combine at Lake Butte des Morts. The Fox River flows east through Oshkosh and into Lake Winnebago at its west central shore, then flows out at the northwest shore, around Doty Island at Neenah-Menasha to Little Lake Butte des Morts. The river then flows northeast and empties into Green Bay and serves as part of the Fox-Wisconsin Waterway. Lake Winnebago is part of a larger system of lakes in Wisconsin known as the Winnebago Pool.

Cities on its shores include the aforementioned Oshkosh, Neenah, and Menasha, as well as Fond du Lac at the south shore. Municipal drinking water systems serving over 200,000 people draw directly from the lake, including systems serving the cities of Oshkosh, Neenah, Menasha, and Appleton.