Oystermouth (Welsh: Ystumllwynarth) is a village (and former electoral ward) in the district of Mumbles, Swansea, Wales.

Morgan derives this name from Ystum Lluarth which he defined as a "place of entrenchment on a hill" adding that the English name Oystermouth, is both a corruption of the existing Welsh name and a folk etymology connected to the locality's famous oyster beds.

[3] When the parish church was restored in 1860, fragments of Roman tessellated pavement were found in various parts of the churchyard.

[4] The name "Caer Tawy" would indicate the existence of a Welsh-built fortification here predating the construction of both Norman castles at Oystermouth and Swansea.

From the Mumbles Head area, there are views towards Swansea, Port Talbot, and the hills of the South Wales Coalfield.

It was rebuilt in stone by the de Braoses, who were Lords of the Gower, and visited briefly by King Edward I of England in 1284.

[8] Until 2022 the electoral ward consisted of some or all of the following areas: Oystermouth, the Mumbles, Thistleboon, Limeslade, in the parliamentary constituency of Gower.