Thomas Cranmer (2 July 1489 – 21 March 1556) was a leader of the English Reformation and Archbishop of Canterbury during the reigns of Henry VIII, Edward VI and, for a short time, Mary I. He helped build the case for the annulment of Henry's marriage to Catherine of Aragon, which was one of the causes of the separation of the English Church from union with the Holy See. Along with Thomas Cromwell, he supported the principle of royal supremacy, in which the king was considered sovereign over the Church within his realm.

During Cranmer's tenure as Archbishop of Canterbury, he was responsible for establishing the first doctrinal and liturgical structures of the reformed Church of England. Under Henry's rule, Cranmer did not make many radical changes in the Church, due to power struggles between religious conservatives and reformers. He published the first officially authorised vernacular service, the Exhortation and Litany.

When Edward came to the throne, Cranmer was able to promote major reforms. He wrote and compiled the first two editions of the Book of Common Prayer, a complete liturgy for the English Church. With the assistance of several Continental reformers to whom he gave refuge, he changed doctrine or discipline in areas such as the Eucharist, clerical celibacy, the role of images in places of worship, and the veneration of saints. Cranmer promulgated the new doctrines through the Prayer Book, the Homilies and other publications.

After the accession of the Catholic Mary I, Cranmer was put on trial for treason and heresy. Imprisoned for over two years and under pressure from Church authorities, he made several recantations and apparently reconciled himself with the Catholic Church. While this would have normally absolved him, Mary wanted him executed, and, on the day of his execution, he withdrew his recantations, to die a heretic to Catholics and a martyr for the principles of the English Reformation. Cranmer's death was immortalised in Foxe's Book of Martyrs and his legacy lives on within the Church of England through the Book of Common Prayer and the Thirty-Nine Articles, an Anglican statement of faith derived from his work.