Sir Diarmaid Ninian John MacCulloch (; born 31 October 1951) is a British historian and academic, specialising in ecclesiastical history and the history of Christianity. Since 1995, he has been a fellow of St Cross College, Oxford; he was formerly the senior tutor. Since 1997, he has been Professor of the History of the Church at the University of Oxford.
Though ordained a deacon in the Church of England, he declined ordination to the priesthood because of the Church's attitude to homosexuality. In 2009 he encapsulated the evolution of his religious beliefs: "I was brought up in the presence of the Bible, and I remember with affection what it was like to hold a dogmatic position on the statements of Christian belief. I would now describe myself as a candid friend of Christianity." MacCulloch sits on the editorial board of the Journal of Ecclesiastical History.