Pontsticill Reservoir or Taf Fechan Reservoir is a large reservoir on the Taf Fechan lying partly in the county of Powys and partly within the county borough of Merthyr Tydfil in south Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark.
The 110 ft high embankment has, since its completion in 1927, been holding back 3400 million gallons of water for supply to industry and population to the south. The modern reservoir incorporates the earlier Pentwyn Reservoir (sometimes referred to as Dol-y-gaer Reservoir or Lake) which suffered major water losses after completion due to the presence of major fractures in the bedrock beneath its dam relating to the Neath Disturbance, a major geological fault which runs northeast to southwest through the area.
The reservoir is popular with sailors, anglers and picnickers. The Taff Trail has been developed for walkers and cyclists and runs through the woods on the western side of the Reservoir. Merthyr Tydfil sailing club is based on the eastern bank and the Brecon Mountain Railway which runs up its eastern side from Pant Station to Dolygaer with plans to extend its run to Torpantau. This steam railway runs on the route of the former Brecon and Merthyr Railway.
Most of the banks of the reservoir have been heavily afforested by Welsh Water though management of these woods is undertaken by the Forestry Commission on behalf of the company.
Part of the 2011 film Submarine was filmed on the reservoir walkway.