Louis IX (25 April 1214 – 25 August 1270), commonly known as Saint Louis or Louis the Saint, was King of France from 1226 to 1270, and the most illustrious of the Direct Capetians. He was crowned in Reims at the age of 12, following the death of his father Louis VIII. His mother, Blanche of Castile, ruled the kingdom as regent until he reached maturity, and then remained his valued adviser until her death. During Louis' childhood, Blanche dealt with the opposition of rebellious vassals and secured Capetian success in the Albigensian Crusade, which had started 20 years earlier.

As an adult, Louis IX faced recurring conflicts with some of his realm's most powerful nobles, such as Hugh X of Lusignan and Peter of Dreux. Simultaneously, Henry III of England attempted to restore the Angevin continental possessions, but was promptly routed at the Battle of Taillebourg. Louis annexed several provinces, notably parts of Aquitaine, Maine and Provence.

Louis IX enjoyed immense prestige throughout Christendom and was one of the most notable European monarchs of the Middle Ages. His reign is remembered as a medieval golden age in which the Kingdom of France reached an economic as well as political peak. His fellow European rulers esteemed him highly for his skill at arms, the power and unmatched wealth of his kingdom, but also for his reputation for fairness and moral integrity; he was often asked to arbitrate their disputes.He was a reformer and developed a process of French royal justice in which the king was the supreme judge to whom anyone could in theory appeal for the amendment of a judgment. He banned trials by ordeal, tried to end the scourge of private wars, and introduced the presumption of innocence to criminal procedures. To enforce his new legal system, Louis IX created provosts and bailiffs.

Honoring a vow he had made while praying for recovery during a serious illness, Louis IX led the ill-fated Seventh Crusade and Eighth Crusade against the Muslim dynasties that ruled North Africa, Egypt and the Holy Land in the 13th century. He was captured in the first and ransomed, and he died from dysentery during the latter. He was succeeded by his son Philip III.

His admirers through the centuries have regarded Louis IX as the ideal Christian ruler. He was a splendid knight whose kindness and engaging manner made him popular, though contemporaries occasionally rebuked him as a "monk king". He was seen as inspired by Christian zeal and Catholic devotion. Enforcing strict Catholic orthodoxy, his laws punished blasphemy by mutilation of the tongue and lips, and he ordered the burning of some 12,000 manuscript copies of the Talmud and other important Jewish books after the Disputation of Paris of 1240. He is the only canonized king of France, and there are consequently many places named after him.