Cefn yr Ystrad is a mountain in the Brecon Beacons National Park in Wales.

The rock strata generally dip to the south but are locally disrupted by foundering of the sandstone as the underlying limestone has dissolved away.

The crest and southern slopes of the hill are home to dozens of shakeholes, some of which reach considerable proportions.

[3] Numerous nineteenth century boundary stones can traced across the hill bearing the engraved letters ‘D of B, TM’ on the one side and ‘GH’ on the other, marking the boundary between the estates of the Duke of Beaufort, Tretower Manor and of the Gwynne Holford’s who were established at Buckland Hall near Bwlch[4] A number of more recent structures and tracks are associated with the now-abandoned limestone quarries of Cwar Blaen-dyffryn, Cwar y Hendre and Cwar yr Ystrad on the northern and northwestern flanks of the hill.

A bridleway running northeast from Pontsticill skirts the northwestern edge of the hill bound for Dyffryn Crawnon and the Usk Valley.