St Caian's Church, Tregaian, also known as St Caean's Church, Tregaean, is a small medieval church dating from the 14th century in Anglesey, north Wales. It is dedicated to , a Christian from the 5th or 6th century about whom little is known. The building contains a late 14th-century east window and a late 15th-century doorway. The churchyard contains the grave of William ap Howel, who died in 1581 at the age of 105, leaving over forty children between the ages of 8 and 89 and over three hundred living descendants.