The Duchy of Brittany (Breton: Dugelezh Breizh, [dyls brjs]; French: Duch de Bretagne) was a medieval feudal state that existed between approximately 939 and 1547. Its territory covered the northwestern peninsula of Europe, bordered by the Atlantic Ocean to the west, and the English Channel to the north. It was also less definitively bordered by the river Loire to the south, and Normandy, and other French provinces, to the east. The Duchy was established after the expulsion of Viking armies from the region around 939. The Duchy, in the 10th and 11th centuries, was politically unstable, with the dukes holding only limited power outside their own personal lands. The Duchy had mixed relationships with the neighbouring Duchy of Normandy, sometimes allying itself with Normandy, and at other times, such as the Breton-Norman War, entering into open conflict.

Henry II of England invaded Brittany in the mid-12th century and became Count of Nantes in 1158 under a treaty with Duke Conan IV. Henry's son, Geoffrey, became Duke through his marriage to Constance, the hereditary Duchess. The Angevins remained in control until the collapse of their empire in northern France in 1204. The French Crown maintained its influence over the Duchy for the rest of the 13th century. Monastic orders supported by the Breton aristocracy spread across the Duchy in the 11th and 12th centuries, and in the 13th, the first of the mendicant orders established themselves in Brittany's major towns. Civil war broke out in the 14th century, as rival claimants for the Duchy vied for power during the Breton War of Succession, with different factions supported by England and France.

The independent sovereign nature of the Duchy began to come to an end upon the death of Francis II in 1488. The Duchy was inherited by his daughter, Anne, but King Charles VIII of France had her existing marriage annulled and then married her himself. As a result, the King of France acquired the title of Duke of Brittany jure uxoris. The Ducal crown became united with the French crown in 1532 through a vote of the Estates of Brittany, after the death of Queen Claude of France, the last sovereign duchess. Her sons Francis III, Duke of Brittany and then Henry II of France would in any case have created a personal union on the death of their father.

Following the French Revolution, and as a result of the various republican forms of French government since 1792, the duchy was replaced by the French system of dpartements (or departments) which continues under the Fifth Republic of France. In modern times the departments have also joined into administrative regions although the administrative region of Brittany does not encompass the entirety of the medieval duchy.

The union of Brittany and France was a critical step in the formation of modern-day France. Brittany had been a semi-independent component of the Kingdom of France since Clovis I was given authority over the Gallo-Roman domain during the 5th century. It was first recorded as a "duchy" during the rule of Nominoe in 846. Over the centuries, the fealty demonstrated by the Duchy of Brittany toward the French king depended significantly on the individuals holding the two titles, as well as the involvement of the English monarchy at that particular time. The reign of Francis II, Duke of Brittany, was at an especially crucial time, as the nobles struggled to maintain their autonomy against the increasing central authority desired by Louis XI of France. As a result of several wars, treaties, and papal decisions, Brittany was united with France through the eventual marriage of Louis XI's son Charles VIII to the heiress of Brittany, Anne in 1491. However, because of the different systems of inheritance between the two realms, the crown and the duchy were not held by the same hereditary claimant until the reign of Henry II, beginning 1547.