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.

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