Saint Cybi (in Welsh) or Saint Cuby (in Cornish) was a 6th century Cornish bishop, saint and, briefly, king, who worked largely in North Wales. His biography is recorded in two slightly variant medieval 'lives'.

Life in Cornwall
According to the 'Life of Saint Cybi', this saint was the son of Salomon, a 'warrior prince', generally thought to have been a King of Cornwall. In the 'Bonedd y Saint', his father's name is given the Welsh form, Selyf. He was raised as a Christian and, in early life, went on a pilgrimage to Rome and Jerusalem. He became a priest and was consecrated as a bishop, before he arrived home to find that his was father dead and he was King of Cornwall. Cybi politely declined the Throne and, instead, travelled through his kingdom, preaching to the people and building churches at Duloe, Tregony, Cubert and Landulph.

Life in Wales
He then moved on to south Wales, founding churches at Llangybi-ar-Wysg near Caerleon, and Llanddyfrwyr-yn-Edeligion, before spending some time in Ireland. He eventually sailed for north Wales, settling at Llangybi on the Llŷn peninsula. Off Anglesey, King Maelgwn of Gwynedd gave him the old Roman fort at Holyhead (subsequently known, in Welsh, as Caergybi, "Cybi's Fort") on Holy Island (thence called Ynys Gybi, "Cybi's Island"). He founded a large and important monastery there.

Cybi was a great friend of Saint Seiriol. He would always face the Sun while travelling to meet him and his resulting tanned complexion led to him being known as Cybi Felyn or Cybi the Tanned. He is also said to have attended the Synod of Llanddewi Brefi in Ceredigion where he advised some pilgrims on their journey to Ynys Enlli (Bardsey).

Cybi died on 8 November 555 and was buried in 'Capel y Bedd' (the Chapel of the Grave) adjoining his monastery at Holyhead.