The River Foss is an improved river in North Yorkshire, England, and a tributary of the River Ouse. It rises in the Foss Crooks woods near Oulston reservoir close to the village of Yearsley and runs south through the Vale of York to the Ouse. The name most likely comes from the Roman word Fossa, meaning ditch and is mentioned in the Domesday Book.

The responsibility for the management of the river's drainage area lies with the Foss Internal Drainage Board (IDB). It has responsibility for the area from Crayke to the pre-1991 city boundary of York covering 9,085 hectares and 162.54 km of waterways. The Foss IDB is itself part of the York Consortium of Drainage Boards that oversees 10 IDB's in the Yorkshire region.

The typical river level range at the Foss Barrier is between 5.05m and 7.90m. The highest river level recorded at this location was 10.20 metres and the river level reached 9.34 metres on 23 January 2008.

The source of this river is a spring situated in the Howardian Hills adjacent to, and flowing into, Oulston Reservoir near Newburgh Priory, 4 miles (6.5 km) north of Easingwold. From there to the Blue Bridge in York, where it joins the River Ouse, it is 19.5 miles (31 km) in length. For part of its way it runs close to the B1363 between Brandsby and Stillington. The river flows in a series of wide meanders in southerly direction for most of its course towards York. As of 2010 the river is only navigable for some 1.5 miles (2 km) upstream of Castle Mills Lock. The bridges by Peasholme Green and Foss Bank restrict the headroom to an air space of 2.4 metres.