Decuman (Medieval Latin: Decumanus; Welsh: Degyman; died c. 706) was one of the Celtic saints who came to Somerset from Pembrokeshire, South Wales, in the seventh century, arriving on a raft (or his cloak) with a cow for a companion.

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.

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).

[1] The saint is associated with several other places in south Wales and also with Degibna, on Loe Pool, near Helston in Cornwall, where there was a chapel dedicated to him.