Llwyn-on Reservoir is the largest and southernmost of the three reservoirs in the Taff Fawr valley in South Wales. It is owned by Welsh Water. It is located in the Brecon Beacons National Park. The eastern half is in the Merthyr Tydfil unitary authority area and the western half is in the Rhondda Cynon Taff unitary authority area. The reservoir is within the historic county boundaries of Breconshire. The dam is adjacent to Llwyn-On village.
Nant Gwinau, Nant Car and Garwnant are the major streams that enter the reservoir.
To the east of the reservoir is the 462m mountain of Garn Ddu, and to the west is the 485m mountain of Cadair Fawr.
There are a variety of guided walks and waymarked paths. Environmental sculptures can be found on the Wern and Willow walks. The Taff Trail and the Navvies Line paths link Cefn Coed-y-Cymmer to Brecon. There is one bird hide.