St Peter's is the parish church of Carmarthen and the largest medieval building still in use in the town. The earliest fabric is 13th-century, but the church was extensively remodelled in the 14th and 15th centuries — the chancel arch, the tower and most of the nave windows date from this period. The exterior is restrained Welsh perpendicular; the interior is more rewarding, with a fine 15th-century roof, an Easter Sepulchre, and a set of carved heads on the arcade capitals.
The principal monument is the tomb of Lieutenant-General Sir Thomas Picton, the Carmarthenshire commander killed at Waterloo in 1815. The marble tomb-chest depicts Picton on the battlefield and is one of the more elaborate Napoleonic memorials in Britain. Picton's reputation has been seriously damaged in recent decades by reconsideration of his record as governor of Trinidad, where he authorised and oversaw the torture of a 14-year-old free woman of colour; the church carries an explanatory panel near the tomb.
The church is open most weekday afternoons and on Sundays for services. It is at the top of King Street, a five-minute walk from the market and the castle.