The River Tywi (Welsh) or River Towy (English) is the longest river which entirely runs through Wales.

It rises on lower slopes of Crug Gynan in the Cambrian Mountains .and flows through the Tywi Forest forming the border between Ceredigion and Powys.

The Llyn Brianne Reservoir was formed here in 1972 by damming a section of the river in order to store winter rain so that it could be released into the river during dry periods. The purpose behind this construction was to support the new abstraction at Nant Garedig that was destined to supply a large swathe of south Wales with drinking water. The flow in the River Twyi would have been unable to sustain such an abstraction were it not for the release of water from the upland reservoir.

The river flows south-westwards through Carmarthenshire passing through the towns of Llandovery, Llandeilo. In Carmarthen it is joined by the substantial tributary, the Afon Gwili at Abergwili. Finally the Twyi flows into Carmarthen Bay in a combined estuary with the River Tâf and River Gwendreath , east of the Pendine Sands.

The total length of the River Tywi is 108 km (68 miles). Its numerous tributaries include the River Cothi, River Gwili , River Brân and River Doethie . The Tywi is noted for trout and salmon fishing.