Hay Bluff (Welsh: Penybegwn)[1] is a prominent hill at the northern tip of the Black Mountains, an extensive upland massif which straddles the border between south-east Wales and England.

The flat summit of Hay Bluff which is marked by a triangulation pillar at a height of 677 metres (2,221 ft) overlooks the middle Wye Valley and the book town of Hay-on-Wye.

A number of small quarries have been worked on its slopes in the past, not least for the Ffynnon Limestone, a calcrete which crops out along the northeastern and northwestern flanks of the hill at the base of the Senni Formation.

There is also a clear, but unpaved path which passes along the edge of the escarpment to the east with good views over the countryside towards Hay-on-Wye.

It leads to the trig point at the Black Hill (Herefordshire), and continues on to the south-east dropping down the stony knife-edge ridge towards Longtown.