Deadwater Ait is an island in the River Thames in England on the reach above Romney Lock, near Windsor, Berkshire.
The island is an uninhabited tree-covered strip adjacent to the Windsor bank of the river between Windsor Bridge and Windsor Railway Bridge.
Swans, geese and a small number of coots all live on this island.
The same distance as its length downstream, is Barry Avenue Island, or unnamed, which is slightly larger at , compared to Deadwater's size, as itemised by 25-inch-to-the-mile maps produced, out of copyright, in 1897. It is the same distance from the Windsor bank and has the same use for habitat and wildlife.