Mynydd Allt-y-grug is a 338-metre-high hill immediately west of Ystalyfera in the county borough of Neath Port Talbot in South Wales. Its twin summits are around 400m apart. The upper part of the hill is largely covered in heather whilst conifers clothe its western slopes and gorse, bracken and bramble cover much of its rough landslipped eastern side which rises above the Swansea Valley, drainage on this side flowing to the River Tawe.