The Dee Estuary is a large estuary by means of which the River Dee flows into Liverpool Bay. The estuary starts near Shotton after a five-mile (8 km) 'canalised' section and the river soon swells to be several miles wide forming the boundary between the Wirral Peninsula in north-west England and Flintshire in north-east Wales. The Dee Estuary's largest towns along it include Holywell, Flint, Connah's Quay, Shotton, Queensferry, Saltney Ferry, Heswall, West Kirby and Neston as well as other villages and towns alongside it. The A548 also passes along the estuary in Wales and parts of Cheshire West and Chester and Merseyside in England. The North Wales Coast Line follows the course of the Dee Estuary between Prestatyn and Chester.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.