Saint Mellitus (d. April 24 624) was the first Bishop of London and the third Archbishop of Canterbury.

Bede describes Mellitus as being of noble birth (Hist. Eccl., II, vii) and Pope Gregory I describes him as an abbot (Letters, xi, 54, 59). He may have been abbot of the monastery of St. Andrew on the Celian Hill, founded by Gregory, to which both Gregory belonged before he became pope and St. Augustine belonged before he was sent to Kent.

Mellitus was sent to England in 601 AD by Gregory in response to an appeal from Augustine, the first Archbishop of Canturbury, for a fresh band of missionaries. He was commissioned by Gregory to convey the pallium to Augustine, together with a present of books and all things which were needed for worship and the ministry of the Church.

Mellitus was consecrated as Bishop of London by Augustine in 604 and founded St. Paul's Cathedral. He was driven from London by the heathen sons of King Sebert of Essex because his refused to give them the sacramental bread unless they consented to be baptised. He fled to Gaul but was recalled by St. Laurence of Canterbury, the second Archbishop of Canterbury; upon Laurence's death in 619, Mellitus succeeded him as the third Archbishop of Canterbury.