Hornsea Mere is a large, shallow freshwater lake on the western edge of the town of Hornsea in East Yorkshire (TA190470). It has a surface area of about 130 ha and a mean depth of about 1.2 m. It is situated at about 3.5 m above sea level, and has a catchment area of about 1755 ha. The centre of the mere is 2.1 km from the North Sea, which lies to the east of the site.
The mere is surrounded by areas of fringing swamp, and its catchment includes grassland, woodland, agricultural land and parts of the town of Hornsea. It has been designated as a Site of Special Scientific Interest (SSSI) in recognition of its conservation interest, which includes aquatic plants, adjacent habitats and wintering and breeding bird populations. It is also a Special Protection Area (SPA) because it hosts the European Protect Species Anas strepera, the Gadwall (dabbling duck).