Southampton Water is a tidal estuary north of the Solent and the Isle of Wight in England. The city of Southampton lies at its most northerly point, where the estuaries of the River Test and River Itchen meet. Along its salt marsh-fringed western shores lie the New Forest villages of Dibden, Hythe and Fawley, and the Fawley Refinery. On the slightly steeper eastern shore are the Southampton suburb of Weston, the villages of Netley and Hamble-le-Rice, and the Royal Victoria Country Park. To the south, Southampton Water enters the Solent between Calshot Spit and Hill Head.

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.