Mynydd Mallaen is an expansive hill to the northwest of Cilycwm in northeast Carmarthenshire, Wales. It takes the form of an undulating plateau with steep slopes dropping away to the Towy valley to the east and those of the Gwenffrwd, Nant Melyn and Afon Cothi to the north and west. Its highest point of 462m at OS grid reference SN 723455 is surmounted by two Bronze Age cairns known as Crugiau Merched (which translates from Welsh as ladies barrows). Caeo Forest covers much of the southern flanks of the hill and smaller forests also decorate its eastern slopes.
The geology of Mynydd Mallaen is complex, comprising mudstones of the Claerwen Group together with sandstones of the Doethie and Glanyrafon Formations and the Caerau Mudstones Formation. Each of these Silurian rock formations is folded and faulted. Indeed it is an outcrop of conglomerate turbidite rock on the western limb of the Cothi Anticline which forms the summit of the hill. Glacial till from the last ice age occupies some of the hoolws on the plateau. 
The entire hill is mapped as open country under the Countryside and Rights of Way Act 2000 so is freely available to those on foot. Additionally there is a bridleway which crosses the hill in an east-west direction and a further one approaching from the south.
- Ordnance Survey 1:25,000 scale Explorer map sheet 187 Llandovery/Llanymddyfri
- British Geological Survey 1:50,000 scale geological map sheet (E&W series) 195 Lampeter