Mynydd Marchywel is a 418-metre (1,371 ft) high hill in the Neath Port Talbot area in South Wales.

The larger part of the hill is cloaked in modern forestry through which numerous streams fall away westward to the River Tawe, eastward to the River Dulais and southward into the Clydach, the latter two being tributaries of the River Neath.

One of the last operations was that at Gleision Colliery under the northwestern side of Mynydd Marchywel.

Mynydd Marchywel is crossed by Saint Illtyd's Walk which locally runs from Ynysmeudwy to Crynant across the northern slopes of the hill, following one of the very few public rights of way to be found on the hill.

There are however numerous forest tracks and most of its western flanks, including the summit area, are designated as open access under the Countryside and Rights of Way Act 2000.