The Château de Josselin developed from a fortress controlling the Oust valley into the enduring seat of the Rohan family. Around the beginning of the eleventh century Guéthénoc, Viscount of Porhoët, established a stronghold on the rocky height above the river. Its position guarded routes through central Brittany and encouraged a town to grow below. The early castle became entangled in the struggles between Breton lords and the rulers who sought to dominate the duchy. In 1168 forces of Henry II of England destroyed the fortress during conflict with the Porhoët family.

The site was rebuilt, but its most important medieval transformation followed Olivier V de Clisson's acquisition of Josselin in the fourteenth century. Clisson, constable of France and one of the most powerful soldiers of the Hundred Years' War, constructed a massive new fortress after about 1370. Nine towers and formidable walls made it a statement of military and political strength. Clisson's loyalties shifted during the rivalries surrounding the Breton succession, and Josselin served as both residence and strategic base.

Through the marriage of Clisson's daughter Béatrix to Alain VIII de Rohan, the castle passed to the Rohans. Their power sometimes brought confrontation with the dukes of Brittany. Jean II de Rohan supported the French crown against Duchess Anne. After the French victory at Saint-Aubin-du-Cormier in 1488, the political balance changed, but royal authority also checked overmighty nobles. Parts of Josselin's fortifications were dismantled. In the early sixteenth century Jean II commissioned the elegant residential range facing the courtyard, whose carved granite façade joined late Gothic ornament with Renaissance ideas while the river side retained its great medieval towers.

Religious conflict again made the castle politically dangerous. The Rohan family became prominent Protestant leaders during the French Wars of Religion and the Huguenot rebellions. Henri, Duke of Rohan, opposed the centralizing government of Louis XIII. After royal victory, Cardinal Richelieu ordered sections of Josselin's defenses demolished in 1629. The keep and several towers disappeared, reducing the fortress's military capacity while leaving the surviving river front as a reminder of its former scale.

During the French Revolution the château was seized, stripped, and used for practical purposes including storage and detention. The family later recovered the property, but extensive repair was necessary. In the nineteenth century the Rohans undertook major restoration, part of a broader revival of interest in medieval Brittany. Interiors were refurnished, damaged architecture repaired, and the château again became a family residence rather than a ruined feudal stronghold.

Josselin's history records the repeated negotiation between regional nobility and central power. English royal forces, Breton succession struggles, French kings, Richelieu's campaigns against Protestant autonomy, and the Revolution all altered or attacked the castle. Yet the site's continued association with the Rohans allowed it to survive those ruptures. Its contrasting façades—fortified towers above the Oust and intricate domestic stonework around the courtyard—embody the passage from medieval warfare to aristocratic residence and modern heritage. The town and castle remained mutually dependent: markets, pilgrimage to the nearby basilica, river traffic, and household employment tied the Rohan residence to local life. Modern public opening and conservation continue that relationship, while the family's occupation preserves a rare continuity often broken at aristocratic properties by revolution or sale.