Muscular Christianity is a religious movement that originated in England in the mid-19th century, characterized by a belief in patriotic duty, discipline, self-sacrifice, masculinity, and the moral and physical beauty of athleticism. The movement came into vogue during the Victorian era as a method of building character in pupils at English public schools. It is most often associated with English author Thomas Hughes and his 1857 novel Tom Brown's School Days, as well as writers Charles Kingsley and Ralph Connor.

American President Theodore Roosevelt was raised in a household that practised Muscular Christianity and was a prominent adherent to the movement. Roosevelt, Kingsley, and Hughes promoted physical strength and health as well as an active pursuit of Christian ideals in personal life and politics. Muscular Christianity has continued through organizations that combine physical and Christian spiritual development. It is influential within both Catholicism and Protestantism.

Until the Age of Enlightenment, the aesthetics of the body within Christianity were concerned chiefly with holy suffering. Asceticism and the denial of bodily needs and beauty were of interest to laity and clergy alike in ancient history and the Middle Ages. A key tenet of asceticism is believing the flesh to be a distraction from divinity. Sects such as Catharism believed the flesh to be wholly corrupted.

The Muscular Christianity movement was never officially organized. Instead, it was a cultural trend that manifested in different ways and was supported by various figures and churches. Muscular Christianity can be traced back to Paul the Apostle, who used athletic metaphors to describe the challenges of a Christian life. However, the explicit advocacy of sport and exercise in Christianity did not appear until 1762, when Rousseau's Emile described physical education as important for the formation of moral character.

The term Muscular Christianity became well known in a review by barrister T. C. Sandars of Kingsley's novel Two Years Ago in the February 21, 1857, issue of the Saturday Review. The term had appeared slightly earlier. Kingsley wrote a reply to this review in which he calls the term "painful, if not offensive", but he later used it favorably on occasion.

In addition to the beliefs stated above, Muscular Christianity preached the spiritual value of sports, especially team sports. As Kingsley said, "games conduce, not merely to physical, but to moral health". An article on a popular 19th-century Briton sums it up thus: "John MacGregor is perhaps the finest specimen of Muscular Christianity that this or any other age has produced. Three men seemed to have struggled within his breast—the devout Christian, the earnest philanthropist, the enthusiastic athlete."

Despite having gained some support, the concept was still controversial. For one example, a reviewer mentions "the ridicule which the 'earnest' and the 'muscular' men are doing their best to bring on all that is manly", though he prefers "'earnestness' and 'muscular Christianity'" to 18th-century propriety. For another, a clergyman at Cambridge University horsewhipped another clergyman after hearing that he had said grace without mentioning Jesus because a Jew was present. A commentator said, "All this comes, we fear, of Muscular Christianity."

Kingsley's contemporary Thomas Hughes is credited with helping to establish the main tenets of Muscular Christianity in Tom Brown at Oxford, which are physical manliness, chivalry, and masculinity of character. In Tom Brown at Oxford, Hughes states "The Muscular Christians have hold of the old chivalrous and Christian belief, that a man's body is given to him to be trained and brought into subjection, and then used for the protection of the weak, the advancement of all righteous causes, and the subduing of the earth which God has given to the children of men." The notion of protecting the weak was related to contemporary English concerns over the plight of the poor and Christian responsibility to one's neighbour.