Hennerton Backwater is a narrow backwater of the River Thames on the Berkshire side of the river, on the reach above Marsh Lock near the villages of Shiplake, Oxfordshire and Wargrave, Berkshire. At Wargrave the Thames turns suddenly northwards, and the Hennerton Backwater splits off at the this point to follow the Thames northwards.
Hennerton Backwater leaves the River Thames at an unnamed river island, passing through Willow Marina, and rejoins just below Ferry Eyot. It is navigable by small boats from the downstream end for much of its length, but the bridge carrying Willow Lane over its upstream end has very limited clearance restricting passage to canoes and small dinghies even under favourable conditions.
The island formed by the backwater and the Thames comprises Wargrave Marsh (now mostly drained) and Lashbrook Eyot (no longer distinct), and contains housing, farmland, and the premises of Henley Sailing Club.