The Château de Blain or Château de la Groulais, is a mediaeval castle constructed in the 13th century and heavily remodelled in the 16th, located in the commune of Blain in the Loire-Atlantique département of France. It formed part of the frontier defences of Brittany along with the towns and castles of Vitré, Fougères, Châteaubriant, Ancenis and Clisson.
The castle stands on the side of the Canal de Nantes à Brest. It has 12 towers and covers an area of 4 hectares.
The first castle was constructed by order of Alan IV, Duke of Brittany, around 1108. The fortress passed by marriage to the Clisson family in 1225. Following Olivier Ier de Clisson's revolt against the Duke, the castle was razed in 1260.
Olivier I's son, Olivier II obtained permission from the Duke to rebuild the castle. The Clissons progressively enlarged the castle during the 14th century.
In 1407, the castle became the property of the House of Rohan. Louis, duc de Rohan is buried here.
During the French Wars of Religion, the castle was besieged and set on fire in 1591 during the fighting between the Duke of Mercœur and Jean de Montauban, the knight De Goust. It was restored by Catherine de Parthenay, who installed herself there with her children. In 1628, Henri II de Rohan having become the leader of the Protestant princes, Cardinal Richelieu ordered the dismantling of the castle which thus lost its military role.