St Decumanus's Church, Rhoscrowther, is situated within a rectilinear churchyard, whose southern boundary is delineated by a stream. St Decuman's Well (NPRN 32499) is some 230m to the west-southwest of the church. It is thought to be the location of a bishop's house mentioned as `Llan Degeman' in a 9th-century text of the Welsh Laws. It has been suggested that the churchyard lies within an outer enclosure which is subdivided by springs. A building known as St Deguman's Chapel adjoins the south side of the nave and is thought to represent a capel-y-bedd (a late medieval chapel erected over a founder's grave). In 1998 the church was redundant but still in the ownership of the Church in Wales.

The church is a Grade 1 listed building, considered a good example of a medieval parish church. The church is constructed of limestone and Old Red Sandstone rubble, and consists of 3-bayed nave, 2-bayed chancel, 2-bayed south chapel, north transept with skew passage, 4-storeyed south transeptal tower, north porch, coalhouse north of the nave's west bay, and vestry south of the nave's west bay. The vestry is thought to date to the 13th century, and is a former chapel or cell. The nave and chancel are thought to be 13th- to early 14th-century, and there is painted recess north side of chancel arch.The north and south transepts date to the mid-14th century. The north transept is traditionally known as the Hendleton Chapel and in 1925, the pedestal of a 15th-century shrine was noted in front of the organ. The south chapel is late 14th-century in date and has two tomb recesses in its south, one with a 14th-century female effigy. The porch dates to the mid-16th century and houses a square scalloped 12th- or 13th-century font, reportedly from St Mary's Church, Pwllcrochan (NPRN 300434). The tower is late 16th-century in date, and the former south transept now forms its base. When the upper stages were added, the transept's south and west walls were rebuilt, or refaced and thickened, in large coursed and squared limestone and Old Red Sandstone rubble. The tower has a spiral stair turret which projects from the south-west corner. The north porch is highly decorative, and with its string-courses and a pinnacled parapet, is unlike any other is the region. The coalhouse was constructed in 1864-1908. The church was restored around 1852, and again in 1897, to the designs of F. Wehnet of Milford. The church was reroofed at this time and the tower partially rebuilt. The church was again restored in 1915, when the former chapel or cell was converted to a vestry. The chancel roof, in Arts and Crafts style, dates from this time.

Sources include:
Cadw, Listed Buildings Database
Cambria Archaeology, 2000, Pembrokeshire Churches, gazetteer, 48
Cambria Archaeology, 2003, Early Medieval Ecclesiastical Sites Project, Pembrokeshire gazetteer
Richard Suggett, Painted Temples: Wallpaintings and Rood-screens in Welsh Churches, 1200–1800, (RCAHMW 2021)

RCAHMW 2022