Caersws is a village and community on the River Severn, in the Welsh county of Powys (Montgomeryshire) 5 miles (8 km) west of Newtown, and halfway between Aberystwyth and Shrewsbury. It has a station on the Cambrian Line from Aberystwyth to Shrewsbury. At the 2011 Census, the community had a population of 1,586 – a figure which includes the settlements of Clatter, Llanwnnog and Pontdolgoch. The village itself had a population of slightly over 800.

The River Severn, at 220 miles (354 km) long, is the longest river in Great Britain. It is also the river with the most voluminous flow of water by far in all of England and Wales, with an average flow rate of 107 m3/s (3,800 cu ft/s) at Apperley, Gloucestershire. It rises in the Cambrian Mountains in mid Wales, at an altitude of 2,001 feet (610 m), on the Plynlimon massif, which lies close to the Ceredigion/Powys border near Llanidloes. The river then flows through Shropshire, Worcestershire and Gloucestershire. The county towns of Shrewsbury, Worcester and Gloucester lie on its course.