The Usk Reservoir (Welsh: Cronfa Wysg) is located in the upper Usk Valley, at 1,006 feet (307 m) above sea level, in the western part of the Brecon Beacons National Park, Wales.

The county boundary between Carmarthenshire and Powys runs through the reservoir which lies about six miles to the north of the Black Mountain escarpment.

It can be located just off the minor road between Trecastle and Llanddeusant, 7 miles (11 km) west of Sennybridge.

The reservoir is now owned by Welsh Water and the surrounding forests are managed on their behalf by Natural Resources Wales.

Most of the area is open to walkers and cyclists via forest tracks and other footpaths.