Thomas Cranmer (2 July 1489 – 21 March 1556) was a theologian, 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 is honoured as a martyr in the Church of England.

Cranmer 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 and protector of his people from the abuses of Rome. During Cranmer's tenure as Archbishop of Canterbury, he established 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 (English) service, the Exhortation and Litany.

When Edward, who was devout and had been raised in the tenets of a reformed Church, came to the throne, Cranmer was able to promote faster changes. He wrote and compiled the first two editions of the Book of Common Prayer, a complete liturgy for the English Church, turning to the language of the people. 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 state and Church authorities, he made several recantations and reconciled himself with the Catholic Church. While this would have customarily absolved him from the heresy charge, Mary wanted him executed on the treason charge, and he was burned at the stake on 21 March 1556; on the day of his execution, he publicly 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.

Cranmer was born in 1489 at Aslockton in Nottinghamshire, England. He was a younger son of Thomas Cranmer by his wife Agnes Hatfield. Thomas Cranmer was of modest wealth but was from an established armigerous gentry family which took its name from the manor of Cranmer in Lincolnshire. A ledger stone to one of his relatives in the Church of St John of Beverley, Whatton, near Aslockton is inscribed as follows: Hic jacet Thomas Cranmer, Armiger, qui obiit vicesimo septimo die mensis Maii, anno d(omi)ni. MD centesimo primo, cui(us) a(n)i(ma)e p(ro)p(i)cietur Deus Amen ("here lies Thomas Cranmer, Esquire, who died on the 27th day of May in the year of our lord 1501, on whose soul may God look upon with mercy"). The arms of the Cranmer and Aslockton families are displayed. The figure is that of a man with flowing hair and gown and a purse on his right side. Their oldest son, John Cranmer, inherited the family estate, while Thomas and his younger brother, Edmund, were placed on the path to clerical careers.

Historians know nothing definite about Cranmer's early schooling. He probably attended a grammar school in his village. At 14, two years after his father's death, he was sent to the newly created Jesus College, Cambridge. It took him eight years to attain his Bachelor of Arts degree, following a curriculum of logic, classical literature, and philosophy. During this time, he began to collect medieval scholastic books, which he preserved faithfully throughout his life. For his master's degree he studied the humanists Jacques Lefèvre d'Étaples and Erasmus. He finished the course in three years. Shortly after receiving his Master of Arts degree in 1515, he was elected to a fellowship of Jesus College.

Sometime after Cranmer took his MA, he married a woman named Joan. Although he was not yet a priest, he was obliged to give up his fellowship, resulting in losing his residence at Jesus College. To support himself and his wife, he took a job as a reader at Buckingham Hall (later reformed as Magdalene College). When Joan died during her first childbirth, Jesus College showed its regard for Cranmer by reinstating his fellowship. He began studying theology, and by 1520, he had been ordained, the university already having named him as one of its preachers. He received his Doctor of Divinity degree in 1526.

Not much is known about Cranmer's thoughts and experiences during his three decades at Cambridge. Traditionally, he has been portrayed as a humanist whose enthusiasm for biblical scholarship prepared him to adopt Lutheran ideas, which were spreading during the 1520s. A study of his marginalia reveals an early antipathy to Martin Luther and an admiration for Erasmus. When Cardinal Wolsey, the King's Lord Chancellor, selected several Cambridge scholars, including Edward Lee, Stephen Gardiner and Richard Sampson, to be diplomats throughout Europe, Cranmer was chosen for an embassy to Charles V, Holy Roman Emperor. His supposed participation in an earlier embassy to Spain mentioned in the older literature, has proved to be spurious.