Decuman is said to have been born of noble parents at Rhoscrowther in Pembrokeshire, Wales where the church is dedicated to him. His name comes from the Latin for a farmer of tithes, which is a smallholder who paid a rent for his farm.
He had a chapel at nearby Pwllcrochan. Wishing to escape from worldly companions he crossed the Bristol Channel and landed at Dunster: he then became a hermit at nearby Watchet, living from the produce of his cow. There he healed the sick and acted as a pastor to the inhabitants.
There he is said to have been killed by a pagan. Legend tells that the holy man's head was cut off by "a certain man more venomous than an asp, more poisonous than the adder...." Beheading is a legend which is found associated with several Celtic saints, but in this variant, the saint miraculously picks up his head, washes it, and replaces it. After this the local people assisted Decuman to build the church (Legends and Folklore of Watchet — Ben Norman).
Decuman is said to have died in AD 706, though this seems rather late for a Celtic saint. His feast day was celebrated at Somerset on 27 February.. The saint is associated with several other places in south Wales and also with Degibma, on Loe Pool, near Helston in Cornwall, where there was a chapel dedicated to him.