The River Monnow (Welsh: Afon Mynwy) marks the England–Wales border for much of its 42 miles (68 km) length.

After flowing through southwest Herefordshire, England, and eastern Monmouthshire, Wales, its confluence with the River Wye is approximately 1⁄3 mile (0.54 km) south of Monmouth.

The Monnow rises near Craswall on Cefn Hill just below the high Black Mountains, Wales.

(myn means swift, and wy is one of the many Welsh words for water) At one time the river was noted for its substantial brown trout population, with the length from Pontrilas to Skenfrith producing record catches.

A recently opened fish pass at Monmouth allowing migratory fish to by-pass a previously impassable weir has once more given Atlantic salmon and sea trout access to the majority of the catchment and salmon were seen jumping at the now dismantled Kentchurch weir for the first time for many years in the Autumn of 2008.