Cefn yr Ystrad is a mountain in the Brecon Beacons National Park in Wales. It is an outlier of the Central Beacons group. The broad northeast - southwest aligned ridge reaches an elevation of 617 metres. One of the southernmost peaks in the Brecon Beacons, it rises to the east of Pontsticill Reservoir. The summit area is a great stretch of wild moorland, with the highest point marked by a trig point.
The hill is formed from successive layers of Carboniferous Limestone and the overlying Twrch Sandstone (formerly known as the Basal Grit of the Millstone Grit). There are extensive ice-smoothed pavements of the latter and loose rock abounds. The crest and southern slopes of the hill are home to dozens of shakeholes, some of which reach considerable proportions.