The River Tâf is a river that rises in the Preseli Hills of North Pembrokeshire, West Wales, near the village of Crymych and is around 50 km (30 mi) long.
Water drains from high ground above the village, and at one time flowed at ground level across the main Cardigan–Tenby road (A478) before falling to the level of the defunct Whitland and Cardigan Branch Line railway station "Crymmych Arms" (Great Western Railway) where, on the UK Ordnance Survey map of 1866 it is shown as the source of the Tâf.
At this point the stream fulfils its description "Crymych" (in Welsh "crooked stream") by turning through almost a right angle along the floor of the valley. The stream is mentioned—with various spellings—in records since 1468 and provided both the village and its hostelry with an identity. From there the Tâf, augmented by numerous minor tributaries, flows ESE to Llanfyrnach then SW through Glandwr, Llanglydwen and Login to Whitland. It then flows east to St Clears, south of which its broad estuary meets Carmarthen Bay near Laugharne. Its length is about 56 km (35 mi) from Crymych to Ginst Point, of which about 14 km (9 mi) is tidal.