Mynydd Bach Trecastell is a hill on the border between the counties of Carmarthenshire and Powys in southwest Wales. It lies within the Black Mountain (range) of the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit is plateau-like and reaches a height of 412m at OS grid ref SN 827312 at Y Pigwn. The name signifies the 'little hill of castle town'.