The B4331 is a short B-road in western Pembrokeshire. It was originally unclassified but had become part of the B4330 by 1932. It gained its present number in 1935 and is unchanged since then.
The road starts at a crossroads on the A487. West is the village of Mathry and the B4331 heads east. We descend across the fields to pass through Castlemorris and cross a few small streams, which join to form the Western Cleddau. After going under the railway line to Fishguard we reach Letterston. The B4331 heads east along St David's Road through the village centre and ends at a crossroads on the A40.