Josselin Castle is a medieval castle at Josselin, in the Morbihan department of Brittany, France, first built in 1008 by Guéthénoc, viscount of Porhoët. The town and castle were named after Guéthénoc's son, Goscelinus, and rebuilt at various times since. The current castle was built by Olivier V de Clisson after 1370. He had acquired the land as part of the dowry on his marriage to Margaret of Rohan. It has been designated as a monument historique since 1928.

The noble Breton family line of Porhoët was a French noble family in the middle ages, from around 990 through the 1200's. Notable members included Odo I, Odo II, and Alan I. Alan I would go on to form the Franco-Breton House of Rohan, which survives to the modern day.