Louis IX (25 April 1214 – 25 August 1270), commonly Saint Louis, was King of France from 1226 until his death. He was also styled Louis II, Count of Artois, from 1226 to 1237. Born at Poissy, near Paris, he was the sixth-great-grandson of Hugh Capet, and thus a member of the House of Capet, and the son of Louis VIII and Blanche of Castile. He worked with the Parliament of Paris in order to improve the professionalism of his administration in regards to legal actions.

He is the only canonised king of France; consequently, there are many places named after him, most notably St. Louis, Missouri, in the United States, São Luís do Maranhão, Brazil, and both the state and city of San Luis Potosí, in Mexico. Saint Louis was also a tertiary of the Order of the Holy Trinity and Captives (known as the Trinitarians). On 11 June 1256, the General Chapter of the Trinitarian Order formally affiliated Louis IX at the famous monastery of Cerfroid, which had been constructed by Felix of Valois north of Paris.

Much of what is known of Louis's life comes from Jean de Joinville's famous biography of Louis, Life of Saint Louis. Joinville was a close friend, confidant, and counsellor to the king, and also participated as a witness in the papal inquest into Louis' life that ended with his canonisation in 1297 by Pope Boniface VIII. Two other important biographies were written by the king's confessor, Geoffrey of Beaulieu, and his chaplain, William of Chartres. The fourth important source of information is William of Saint-Pathus' biography, which he wrote using the papal inquest mentioned above. While several individuals wrote biographies in the decades following the king's death, only Jean of Joinville, Geoffrey of Beaulieu, and William of Chartres wrote from personal knowledge of the king.

Louis was born in 1214 at Poissy, near Paris, the son of King Louis VIII and Blanche of Castile. A member of the House of Capet, Louis was twelve years old when his father died on 8 November 1226. He was crowned king within the month at Reims cathedral. Because of Louis's youth, his mother ruled France as regent during his minority. His younger brother Charles I of Sicily (1227 – 85) was created count of Anjou, thus founding the second Angevin dynasty.

No date is given for the beginning of Louis's personal rule. His contemporaries viewed his reign as co-rule between the king and his mother, though historians generally view the year 1234 as the year in which Louis began ruling personally, with his mother assuming a more advisory role. She continued as an important counselor to the king until her death in 1252.

On 27 May 1234, Louis married Margaret of Provence (1221 – 21 December 1295), whose sister Eleanor was the wife of Henry III of England.

When he was 15, Louis' mother brought an end to the Albigensian Crusade in 1229 after signing an agreement with Count Raymond VII of Toulouse that cleared the latter's father of wrong-doing. Raymond VI of Toulouse had been suspected of murdering a preacher on a mission to convert the Cathars.

Louis's piety and kindness towards the poor was much celebrated. He went on two crusades, in his mid-30s in 1248 (Seventh Crusade) and then again in his mid-50s in 1270 (Eighth Crusade). He had begun with the rapid capture of the port of Damietta in June 1249, an attack which did cause some disruption in the Muslim Ayyubid empire, especially as the current sultan was on his deathbed. But the march from Damietta towards Cairo through the Nile River Delta went slowly. During this time, the Ayyubid sultan died, and a sudden power shift took place, as the sultan's slave wife Shajar al-Durr set events in motion which were to make her Queen, and eventually place the Egyptians' slave army of the Mamluks in power. On 6 April 1250 Louis lost his army at the Battle of Fariskur and was captured by the Egyptians. His release was eventually negotiated, in return for a ransom of 400,000 livres tournois (at the time France's annual revenue was only about 250,000 livres tournois, so it was necessary to obtain a loan from the Templars), and the surrender of the city of Damietta.

Following his release from Egyptian captivity, Louis spent four years in the Crusader kingdoms of Acre, Caesarea, and Jaffa. Louis used his wealth to assist the Crusaders in rebuilding their defences and conducting diplomacy with the Islamic powers of Syria and Egypt. Upon his departure from the Middle East, Louis left a significant garrison in the city of Acre for its defence against Islamic attacks. The historic presence of this French garrison in the Middle East was later used as a justification for the French Mandate.

Louis exchanged multiple letters and emissaries with Mongol rulers of the period. During his first crusade in 1248, Louis was approached by envoys from Eljigidei, the Mongol ruler of Armenia and Persia. Eljigidei suggested that King Louis should land in Egypt, while Eljigidei attacked Baghdad, in order to prevent the Saracens of Egypt and those of Syria from joining forces. Louis sent André de Longjumeau, a Dominican priest, as an emissary to the Great Khan Güyük Khan in Mongolia. However, Güyük died before the emissary arrived at his court, and nothing concrete occurred. Louis dispatched another envoy to the Mongol court, the Franciscan William of Rubruck, who went to visit the Great Khan Möngke Khan in Mongolia.

Louis' patronage of the arts drove much innovation in Gothic art and architecture, and the style of his court radiated throughout Europe by both the purchase of art objects from Parisian masters for export and by the marriage of the king's daughters and female relatives to foreign husbands and their subsequent introduction of Parisian models elsewhere. Louis' personal chapel, the Sainte - Chapelle in Paris, was copied more than once by his descendants elsewhere. Louis most likely ordered the production of the Morgan Bible, a masterpiece of mediaeval painting.

Saint Louis ruled during the so-called "golden century of Saint Louis", when the kingdom of France was at its height in Europe, both politically and economically. The king of France was regarded as a primus inter pares among the kings and rulers of the continent. He commanded the largest army, and ruled the largest and most wealthy kingdom of Europe, a kingdom which was the European centre of arts and intellectual thought (La Sorbonne) at the time. The prestige and respect felt in Europe for King Louis IX was due more to the attraction that his benevolent personality created rather than to military domination. For his contemporaries, he was the quintessential example of the Christian prince, and embodied the whole of Christendom in his person. His reputation of saintliness and fairness was already well established while he was alive, and on many occasions he was chosen as an arbiter in the quarrels opposing the rulers of Europe.

Shortly before 1256 Enguerrand IV of Coucy arrested and without trial hanged three young squires of Laon whom he accused of poaching in his forest. In 1256 Louis had him arrested and brought to the Louvre by his sergeants. Enguerrand demanded judgment by his peers and trial by battle which was refused by the king because Louis thought it obsolete. Enguerrand was tried, sentenced and ordered to pay 12,000 livres. Part of the money was to pay for masses in perpetuity for the men he had hanged.

The perception of Louis IX as the exemplary Christian prince was reinforced by his religious zeal. Louis was a devout Catholic, and he built the Sainte - Chapelle ("Holy Chapel"), located within the royal palace complex (now the Paris Hall of Justice), on the Île de la Cité in the centre of Paris. The Sainte Chapelle, a perfect example of the Rayonnant style of Gothic architecture, was erected as a shrine for the Crown of Thorns and a fragment of the True Cross, precious relics of the Passion of Jesus. Louis purchased these in 1239 – 41 from Emperor Baldwin II of the Latin Empire of Constantinople, for the exorbitant sum of 135,000 livres (the chapel, on the other hand, cost only 60,000 livres to build).

Louis IX took very seriously his mission as "lieutenant of God on Earth", with which he had been invested when he was crowned in Rheims. Thus, in order to fulfill his duty, he conducted two crusades, and even though they were unsuccessful, they contributed to his prestige. Contemporaries would not have understood if the king of France did not lead a crusade to the Holy Land. In order to finance his first crusade Louis ordered the expulsion of all Jews engaged in usury and the confiscation of their property, for use in his crusade. However, he did not cancel the debts owed by Christians. One-third of the debts was forgiven, but the other two thirds were to be remitted to the royal treasury. Louis also ordered, at the urging of Pope Gregory IX, the burning in Paris in 1243 of some 12,000 manuscript copies of the Talmud and other Jewish books. Such legislation against the Talmud, not uncommon in the history of Christendom, was due to mediaeval courts' concerns that its production and circulation might weaken the faith of Christian individuals and threaten the Christian basis of society, the protection of which was the duty of any Christian monarch. In addition to Louis' legislation against Jews and usury, he expanded the scope of the Inquisition in France. The area most affected by this expansion was southern France where the Cathar heresy had been strongest. The rate of these confiscations reached its highest levels in the years prior to his first crusade, and slowed upon his return to France in 1254.

In all these deeds, Louis IX tried to fulfill the duty of France, which was seen as "the eldest daughter of the Church" (la fille aînée de l'Église), a tradition of protector of the Church going back to the Franks and Charlemagne, who had been crowned by the Pope in Rome in 800. Indeed, the official Latin title of the kings of France was Rex Francorum, i.e. "king of the Franks", and the kings of France were also known by the title "most Christian king" (Rex Christianissimus). The relationship between France and the papacy was at its peak in the 12th and 13th centuries, and most of the crusades were actually called by the popes from French soil. Eventually, in 1309, Pope Clement V even left Rome and relocated to the French city of Avignon, beginning the era known as the Avignon Papacy (or, more disparagingly, the "Babylonian captivity").

During his second crusade, Louis died at Tunis, 25 August 1270, and was succeeded by his son, Philip III. Louis was traditionally believed to have died from the bubonic plague but the cause is thought by modern scholars to have been dysentery. The Bubonic Plague did not strike Europe until 1348, so the likelihood of his contracting and ultimately dying from the Bubonic Plague was very slim.

Christian tradition states that some of his entrails were buried directly on the spot in Tunisia, where a Tomb of Saint - Louis can still be visited today, whereas other parts of his entrails were sealed in an urn and placed in the Basilica of Monreale, Palermo, where they still remain. His corpse was taken, after a short stay at the Basilica of Saint Dominic in Bologna, to the French royal necropolis at Saint - Denis, resting in Lyon on the way. His tomb at Saint - Denis was a magnificent gilt brass monument designed in the late 14th century. It was melted down during the French Wars of Religion, at which time the body of the king disappeared. Only one finger was rescued and is kept at Saint - Denis.

Pope Boniface VIII proclaimed the canonization of Louis in 1297; he is the only French monarch to be declared a saint.

Louis IX is often considered the model of the ideal Christian monarch. Because of the aura of holiness attached to his memory, many kings of France were called Louis, especially in the Bourbon dynasty, which directly descended from one of his younger sons.

The Congregation of the Sisters of Saint Louis is a Roman Catholic religious order founded in 1842 and named in his honor. He is also honored as co-patron of the Third Order of St. Francis, which claims him as a member of the Order. He is also venerated at the Church of England. The cities of San Luis Potosí in Mexico; St. Louis, Missouri; St. Louis, Michigan; Saint-Louis du Sénégal; Saint - Louis in Alsace; as well as Lake Saint-Louis in Quebec, the Mission San Luis Rey de Francia in California and rue Saint Louis of Pondicherry are among the many places named after the king and saint. The Cathedral Saint-Louis in Versailles; the Basilica of St. Louis, King of France, and the Cathedral Basilica of St. Louis, both in St. Louis, Missouri; and the St. Louis Cathedral, New Orleans, were also named for the king. The French royal Order of Saint Louis (1693 – 1790 and 1814 – 1830), the Île Saint-Louis as well as a hospital in the 10th arrondissement of Paris also bear his name. Many places in Brazil called São Luís in Portuguese are named after the French Saint Louis. Port-Louis, the capital city of Mauritius, as well as its cathedral are also named after St. Louis, who is the patron saint of the island. A bas-relief of St. Louis is one of the carved portraits of historic lawmakers that adorns the chamber of the United States House of Representatives. Saint Louis is also portrayed on a frieze depicting a timeline of important lawgivers throughout world history in the Courtroom at the Supreme Court of the United States. A statue of St. Louis by the sculptor John Donoghue stands on the roofline of the New York State Appellate Division Court at 27 Madison Avenue in New York City.