It rises at an altitude of 2,001 feet (610 m) on Plynlimon, close to the Ceredigion/Powys border near Llanidloes, in the Cambrian Mountains of mid Wales. It then flows through Shropshire, Worcestershire and Gloucestershire, with the county towns of Shrewsbury, Worcester and Gloucester on its banks. With an average discharge of 107 m3/s (3,800 cu ft/s) at Apperley, Gloucestershire, the Severn is by far the greatest river in terms of water flow in England and Wales.

The river is usually considered to become the Severn Estuary after the Second Severn Crossing between Severn Beach, South Gloucestershire and Sudbrook, Monmouthshire. The river then discharges into the Bristol Channel which in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The Severn's drainage basin area is 4,409 square miles (11,419 km2), excluding the River Wye and Bristol Avon which flow into the Severn Estuary. The major tributaries to the Severn are the Vyrnwy, Clywedog, Teme, Avon and Stour.