St Piran was a 5th-century Irish saint and abbot and is considered the patron saint of Cornwall. His emblem of a white cross on a black background is used as the flag of Cornwall. 

This is the first record of a chapel at Trethevy, but it seems certain that the chapel stood on a site long considered sacred. It is now a chapel of ease for Tintagel parish.

St Piran is usually associated with west Cornwall, so it is odd that the chapel should be dedicated to him. The Domesday Book mentions that the monks of St Piran (or Pieran) owned the manor of 'Tregrebi', which may possibly have been Tregenver, now a farmstead near Trethevy. If so, it suggests that the chapel was dedicated to St Piran because the monks of St Piran were an important local landowner.

Just below the chapel is a dwelling named St Piran's House. Parts of the house are thought to be 14th century and may have formed part of a small monastery. The field above the chapel is still known as Chapel Meadow.

Very little is known about the chapel's early history, but it fell into decay after the Reformation and was abandoned. For many years it was used as a farm building, and it was not restored to ecclesiastical use until 1941.

The chapel was rebuilt in the early 21st century, but several ancient features remain, including a small lancet window with a trefoil head, set into the east wall, and a piscina against the south wall. In the north wall is a colourful modern stained glass window depicting St Piran against a backdrop of the Rocky Valley, which is perhaps misleading, for there is no indication that the saint actually ever visited the area.

The chapel is an extremely simple rectangular building under a slate roof.