River Great Ouse

Origin near Syresham in Northamptonshire
Mouth King's Lynn
Basin countries England
Length 150 miles (240 km)
Source elevation 115 m (380 feet)

The River Great Ouse is a river in the east of England. It is 150 miles (240 km) long which makes it the major navigation in East Anglia, and the fourth-longest river in the United Kingdom. The lower reaches of the Great Ouse are also known as "Old West River" and "the Ely Ouse". The name Ouse is Celtic or pre-Celtic, and probably means simply "water".

The river has several sources close to the villages of Syresham and Sulgrave in Northamptonshire. It flows through Brackley, Buckingham, Milton Keynes at Stony Stratford, Newport Pagnell, Olney, Bedford, St Neots, Godmanchester, Huntingdon, Hemingford Grey, St Ives, the cathedral city of Ely, Littleport, Downham Market and enters The Wash at King's Lynn.

The Environment Agency is the navigation authority and it attempts to attract more boaters to the river.

The Ouse Washes are an internationally important area for wildlife.