Dr Williams, 61, will leave at the end of December to take up a new role as Master of Magdalene College, Cambridge next January. The Queen, Supreme Governor of the Church of England, has been informed.
His reign has been plagued by bitter rows over gay clergy and women bishops that have left him struggling to prevent the Church from unravelling.
Explaining his reasons for leaving, Dr Williams admitted that "crisis management" was not his "favourite activity" but denied the rows over homosexuality had "overshadowed everything".
But he said: "It has certainly been a major nuisance. But in every job that you are in there are controversies and conflicts and this one isn't going to go away in a hurry. I can't say that it is a great sense of 'free at last'."
Dr Williams said his successor would need the "constitution of an ox and the skin of a rhinoceros." Read more...