The Duddon is a river of north-west England. It rises at a point 1,289 feet (393 m) above sea level near the Three Shire Stone at the highest point of Wrynose Pass. The river descends to the sea over a course of about 15 miles (24 km) before entering the Irish Sea at the Duddon Sands. In total, from source to the westernmost part of Duddon Sands, its length is 43 kilometres (27 mi). For its entire length the Duddon forms the boundary between the historic counties of Lancashire and Cumberland and since local government re-organisation in 1974 the Duddon has been in the ceremonial county of Cumbria.

The Duddon Estuary is the sandy, gritty estuary of the River Duddon that lies between Morecambe Bay and the North Lonsdale coast.