Mynydd Hendre-ddu or Mynydd Hendreddu is a mountain in southern Snowdonia, Wales.

It forms the watershed between Nant Llwydiarth, which flows to the south-west and into the Afon Dulas, and streams flowing into Afon Angell to the north, which flow east into the River Dyfi.

[2] The summit is notable for a set of parallel ridges about 80 centimetres (31 in) across and 130 metres (430 ft) long that run across it.

[2] On the northern slope of the mountain lay the Hendreddu quarry which was connected to Aberangell by the Hendre-Ddu Tramway.

Mynydd Hendre-ddu is one of the Dyfi hills.