The River Vyrnwy (Welsh: Afon Efyrnwy) flows through northern Powys, Wales, before continuing into Shropshire, England, where it joins the River Severn near the village of Melverley. The river runs approximately 40 miles (64 km) in total length, with its source at the base of the Lake Vyrnwy dam, which was constructed in the 1880s. Prior to the dam's construction, the river was fed by numerous streams and rivers draining the surrounding mountains.

The river is paddled frequently by kayakers and canoeists. The upper reaches are predominantly Grade II white water, with several Grade III sections. The most notable of these is the Vyrnwy Gorge, located near the village of Dolanog. Also on the upper river is Dolanog Falls, a man-made weir approximately 20 feet (6 m) in height that requires a portage for both kayaks and canoes — this is a significant hazard that paddlers must plan for.

Below Pontrobert, the character of the river changes considerably. The lower section is rated Grade I and is considered a good touring river due to its relatively placid nature when not in flood. Paddlers should be aware that water levels and conditions can change significantly during flood periods, which would affect the difficulty and safety of the lower river.

The river's Welsh/English border section covers the last 8 miles (12.9 km) of its course, running between Powys and Shropshire before reaching the Severn confluence.