Breydon Water is a large tidal estuary at Great Yarmouth, Norfolk, England within The Broads National Park, Britain's largest protected wetland. It is 5 km long and more than 1.5 km wide in places.

It is the confluence of the Rivers Bure, Yare and Waveney. At Breydon Bridge on the east side of Breydon Water, a channel leads to the North Sea at Gorleston, south of Great Yarmouth. Centuries ago, Breydon Water would have been one large estuary facing the sea.

At low tide, one can see vast areas of mudflats and saltings, all teeming with birds. Since the mid-80s, Breydon Water has been a nature reserve in the care of the Royal Society for the Protection of Birds (RSPB). It had been a popular shooting area for centuries, and the shooting continues, but on a very much reduced scale.

In terms of nature conservation, Breydon Water is the most important estuary on the east coast of England between The Wash and the Thames estuary. It has specially protected status by virtue of being a European Union Special Protection Area (SPA).

In the winter, large numbers of wading birds and wildfowl use it to overwinter, including 12,000 Golden Plovers, 12,000 Wigeons, 32,000 Lapwings and tens of thousands of Bewick's Swans.

There is a bird observation hide at the east end of Breydon Water.