Lake Ellesmere is located in the Canterbury region of the South Island of New Zealand. It is a broad, shallow lake located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow strip of land called Kaitore Spit.

The lake covers an area of 265 km², and is the largest lake in the alluvial fan of the Canterbury Plains.

The rarely used Maori name is Waihora, meaning spreading waters (compare Lake Waihola). The main river that feeds the lake is the Selwyn.