Dei gratia regina fidei defensor

Fidei defensor (feminine: Fidei defensatrix) is a Latin title which translates to Defender of the Faith in English and Défenseur de la Foi in French. The phrase has been used as part of the full style of many monarchs since the early 16th century.

"Defender of the Faith" has been one of the subsidiary titles of the English and later British monarchs since it was granted on 11 October 1521 by Pope Leo X to King Henry VIII of England and Ireland. His wife Catherine of Aragon was also a Defender of the Faith in her own right. The title was conferred in recognition of Henry's book Assertio Septem Sacramentorum (Defense of the Seven Sacraments), which defended the sacramental nature of marriage and the supremacy of the Pope. This was also known as the "Henrician Affirmation" and was seen as an important opposition to the early stages of the Protestant Reformation, especially the ideas of Martin Luther.

Following Henry's decision to break with Rome in 1530 and establish himself as head of the Church of England, the title was revoked by Pope Paul III (since Henry's act was regarded as an attack on "the Faith") and Henry was excommunicated. However, in 1544, the Parliament of England conferred the title "Defender of the Faith" on King Henry VIII and his successors, now the defenders of the Anglican faith, of which they (except the Catholic Mary I) remain the Supreme Governors (formally above the Archbishop of Canterbury as Primate).

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