St John's Church is situated at the junction of The Avenue and Priory street, and was built as a Welsh language church for the parish of St Peter's. Its foundation stone was laid on the 25th June 1889 by Bishop Edwards, St Asaph. It was opened on 15 June 1890 by the Bishop of St Davids.

The church is a Grade II listed building, considered a good example of a well designed, late victorian smaller town church. The church's railings, gate and gate piers (NPRN 418353), made at the nearby Priory foundry, are listed seperately. It is Late Perpendicular in style, and was constucted in 1889-1890, to the designs of Middleton, Protheroe and Phillot, Cheltenham. The church is constructed of rock-faced squared Llanelli stone with Bath stone dressings. It consists of four-bayed nave, two-bayed chancel, north and south aisles, porch, north transept (organ chamber) and bellcote over the chancel arch. There is a total immersion font under the floor at the chancel's west end. The nave has three flat-headed three-light cusped windows to the clerestorey. There are gabled butresses between the nave and chancel. The abled ashlar bellcote has two bell-openings. The carved stone reredos dates to 1921.

Sources include:
Cadw, Listed Buildings Database
Lloyd, T, Orbach, J and Scourfield, R, 2006, The Buildings of Wales: Carmarthenshire and Ceredigion

N Vousden, RCAHMW, 30 January 2013