The Four Sons of Aymon (French: [Les] Quatre fils Aymon, is a medieval tale centring around the four sons of Duke Aymon: the knight Renaud de Montauban, his brothers Guichard, Allard and Richardet, their magical horse Bayard, and their adventures and revolt against Emperor Charlemagne. It is sometimes also referred to as Renaud de Montauban, after its main character. The story became popular throughout Europe, and echoes of the story are still found today in certain folklore traditions.

The oldest extant version of the tale is an anonymous Old French chanson de geste, Quatre Fils Aymon, which dates from the late 12th century and comprises 18,489 alexandrine (12-syllable) verses grouped in assonanced and rhymed laisses (the first 12,120 verses use assonance; critics suggest that the rhymed laisses derive from a different poet). It is one of the longest of all the chansons de geste. Other and later versions of the chanson range from 14,300 to 28,000 verses. Of the dozen extant versions of the chanson, all are anonymous except for one, Histoire des quatre fils Aymon, attributed to Huon de Villeneuve [fr], a 13th-century trouvère. The Renaud chansons de geste were transformed into prose romances in the 14th and 15th centuries, and judging from the number of editions, the prose Quatre Fils Aymon was the most popular romance of chivalry in the late 15th century and first half of the 16th century in France.

The tale is generally included in the Doon de Mayence "cycle" of chansons.

Renaud and his three brothers were sons of Aymon de Dordone (a fictional location in the Ardennes, although the name seems to be related to Dordogne near Montauban). At the Pentecostal feast, Aymon brought them to Paris to be presented to Emperor Charlemagne and Renaud proved himself a worthy combatant in the royal tournament and won the emperor's favour. In most versions of the chanson, the emperor presented him with the magical horse Bayard (in two versions, it is the fairy Oriande who gives it to him). Renaud kills one of Charlemagne's nephews (Bertolai) in a brawl over a chess game and the brothers flee, aided by Bayard who can carry all the brothers on his back and leap across valleys.

The brothers decide to hide in the Ardennes where their cousin, the sorcerer Maugis, can help them. Maugis constructs a castle for them called Montessor on a peak overlooking the Meuse. The brothers are, however, forced to flee from Montessor, and eventually they proceed to Gascony to aid King Yvon in his battles against the Emir Begès. Renaud, thanks to his sword "Froberge" (given to him by Maugis), wins a victory, and in gratitude, the king gives Renaud the castle at Montauban and his sister in marriage.

After a series of adventures, Charlemagne is eventually prevailed upon by the noble paladin Roland to make terms with the brothers: the four brothers are pardoned on condition that Renaud travels to the Holy Land on Crusade, and that their magical horse Bayard is surrendered to Charlemagne. Charlemagne orders that Bayard be drowned by chaining him to a stone and throwing him in a river, but the horse escapes and lives eternally in the woods (in some versions the horse is killed). Renaud, upon his return from the Crusades, discovers his wife has died. After sending his sons to be educated at the court of the emperor, he abandons his home and goes to Cologne, where he helps to construct a church. In the end, he is murdered by resentful workers, but his body is miraculously saved from the river and makes its way magically in a cart back to his brothers.

Charlemagne is portrayed as vengeful and treacherous in these stories; the sympathy of the storyteller is clearly with the four brothers, but ultimately feudal authority is upheld.

Historie van den Vier Heemskinderen, the Dutch translation, dated 1508 and held at LMU Munich gives the following version: