The Afon Marlais is a right-bank tributary of the River Tâf in Pembrokeshire, West Wales.

It flows through the Vale of Lampeter to join the Tâf just west of Whitland.

[2] An indication of the source of the river comes from Ordnance Survey maps which show, a short distance east of Redstone Cross (a junction of the A40 and the B4313 roads), a place called Blaenmarlais in the community of Narberth at 51°48′40″N 4°44′27″W / 51.8112°N 4.7407°W / 51.8112; -4.7407.

[4] Gold grains have been found in the river, the angular nature of some suggesting a local source.

[5] Pant-y-Gorphwys and its bridge, and Pont Fadog, are Royal Commission recorded historic place names.