Bertrand du Guesclin (c. 1320 – 1380) was constable of France from 1370 to 1380. He is often considered as one of the greatest French soldiers of his time and one of the important figures of the Hundred Years' War (1337-1453).

Du Guesclin was born in Motte-Broons, near Dinan, in Brittany. He initially served Charles of Blois in the Breton War of Succession (1341-1364). Charles was supported by the French crown, while his rival was allied with England. In 1356-1357, Du Guesclin held Rennes against English attack.

Entering the service of King Charles V of France on Charles's accession (1364), he won the victory of Cocherel over the forces of King Charles II of Navarre. The victory forced Charles II into a new peace with the French king.

On September 29, 1364, at the battle of Auray, du Guesclin and Charles of Blois were heavily defeated by John V, Duke of Brittany and the English forces under warlord Sir John Chandos. Charles was killed in action, ending the Blois pretentions in Brittany. Du Guesclin was captured. Ransomed by Charles V, who placed him at the head of the "free companies," the marauding soldiers who pillaged France after the Treaty of Brétigny between France and England, Du Guesclin was sent to Spain to aid Henry of Trastamara (later Henry II of Castile) against Peter the Cruel.

Du Guesclin, though successful in the campaign of 1366, was defeated and captured in 1367 by Peter and Edward the Black Prince at Nájera. In 1369, however, he and Henry won the battle of Montiel, gaining for Henry the throne of Castile.

Warfare with England was renewed in 1369, and Du Guesclin reconquered Poitou and Saintonge and pursued the English into Brittany from 1370 to 1374. He disapproved of the confiscation of Brittany by Charles V in 1378, and his campaign to make the duchy submit to the king was halfhearted.

An able tactician and a loyal and disciplined warrior, Du Guesclin had reconquered much of France from the English when he died of dysentery while on a military expedition in Languedoc.