Mellitus (died 24 April 624) is the first Bishop of London in the Saxon period, the third Archbishop of Canterbury, and a member of the Gregorian mission sent to England to convert the Anglo-Saxons from their native paganism to Christianity.

He arrives in 601 with a group of clergymen sent to augment the mission, and is consecrated as Bishop of London in 604.

Mellitus is the recipient of a famous letter from Pope Gregory I known as the Epistola ad Mellitum, preserved in a later work by the medieval chronicler Bede, which suggests the conversion of the Anglo-Saxons be undertaken gradually, integrating pagan rituals and customs.

In 610, Mellitus returns to Italy to attend a council of bishops, and returnsto England bearing papal letters to some of the missionaries.

Mellitus is exiled from London by the pagan successors to his patron, King Sæberht of Essex, following the latter's death around 616.

King Æthelberht of Kent, Mellitus' other patron, dies at about the same time, forcing him to take refuge in Gaul.

Mellitus returns to England the following year, after Æthelberht's successor has been converted to Christianity, but he is unable to return to London, whose inhabitants remain pagan.

Mellitus is appointed Archbishop of Canterbury in 619.

During his tenure, he is alleged to have miraculously saved the cathedral, and much of the town of Canterbury, from a fire.

After his death in 624, Mellitus is revered as a saint.