Southampton Water is a stretch of the sea north of the Isle of Wight and the Solent, in England. The city of Southampton lies at its most northerly point. Along its western shores lie the New Forest villages of Hythe and "the waterside", Dibden Bay , and the Esso oil refinery at Fawley.

Together with the Solent, Southampton Water is world-renowned for yachting.

The rivers Test, Itchen and Hamble flow into Southampton Water.

Southampton's emergence as a major port, and particularly as a port handling very large vessels, depended partly on certain geographical features of Southampton Water. Its depth, even in its undeveloped state, was generous; this depth of water has been been increased over the years with comparative ease since the soft silt of the river-bed allows for easy dredging. An additional factor is the phenomenon of the "double tide", which is caused by the slowing effect of the Isle of Wight on tides entering Southampton Water. This results in unusually prolonged periods of high water which greatly facilitates the movements of very large ships.