Mallwyd is a small village at the most southern end of Gwynedd, Wales, in the Mawddwy community, in the valley of the River Dyfi. It lies on the A470 approximately halfway between Dolgellau and Machynlleth, and forms the junction of the A458 towards Welshpool. The nearest villages are Dinas Mawddwy, two miles to the north, and Aberangell a similar distance to the south. The River Dugoed flows into the River Dyfi near the village. The Cambrian Way long-distance walk passes through the village.

The River Dyfi, also known as the River Dovey, is an approximately 30-mile (48-kilometre) long river in Wales.