The River Foss is an improved river in the unitary authority of City of York and a tributary of the River Ouse. It was promoted by Acts of Parliament in 1793 and 1801. It is navigable for just over a mile from Monks Bridge (over the A1036 road) to Blue Bridge in the centre of York where it joins the Ouse.

The Foss Barrier is built across the river near its mouth. When closed, it prevents floodwater from the much larger Ouse flowing up the Foss and flooding those parts of York in its vicinity. Water flowing down the Foss can be pumped over the top of the barrier and into the Ouse.