Mynydd Marchywel is a 418-metre-high hill in the Neath Port Talbot area in South Wales. Its summit is marked both by a cairn and a trig point. 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.