| Llwyn-onn Reservoir | |
|---|---|
 | |
  Llwyn-onn Reservoir | |
| Location | South Wales |
| Coordinates | 51°47′50″N 3°26′27″W / 51.79722°N 3.44083°W |
| Type | reservoir |
| Primary inflows | Taf Fawr, Nant Gwinau, Nant Car, Garwnant |
| Primary outflows | Taf Fawr |
| Basin countries | United Kingdom |
Llwyn-onn Reservoir is the largest and southernmost of the three reservoirs in the Taf Fawr valley in South Wales. Cardiff Corporation Waterworks obtained an Act of Parliament in 1884 to authorise construction of the reservoirs, to increase the water supply for Cardiff, but construction of Llwyn-onn Reservoir did not start until 1911 and was completed in the 1920s.
The reservoir, which has been owned and managed by Welsh Water and its predecessor organisation, the Welsh Water Authority, since 1973, is located within the Brecon Beacons National Park with its eastern half in the Merthyr Tydfil unitary authority area and the western half in Rhondda Cynon Taf. The village of Llwyn-Onn is nearby and the A470 trunk road runs along its eastern shoreline.