St Cwyllog's Church, Llangwyllog is a medieval church near Llangwyllog, in Anglesey, north Wales. St Cwyllog founded a church here in the 6th century, although the exact date is unknown. The existence of a church here was recorded in 1254 and parts of the present building may date from around 1200. Other parts are from the 15th century, with an unusual annexe (possibly intended for use as a schoolroom) added in the 16th century.