Alain I de Rohan (1084–1147), also known as le Noir, was the 1st Viscount de Rohan and Viscount of Castelnoec. He was the third son of Eudon I de Porhoët, viscount of Porhoët, and Emma de Léon (daughter of Guyomarch II, Viscount de Léon).
He received as his inheritance, the sparsely populated western Porhoët, in 1116. He also owned a castle at Castennec and constructed another on the shores of the Oust which he named Rohan, the name he passed on to his descendants. In 1128, Alain de Rohan finished constructing his permanent residences and founded the Priory de la Coarde at Castennec for the monks at Redon Abbey, and a priory for Marmoutier Abbey near the château de Rohan.
He had three children:
The House of Rohan is a French noble family of viscounts, later dukes and princes, coming from the locality of Rohan in Brittany. Their line descends from the viscounts of Porhoët and is said to trace back to the legendary Conan Meriadoc. Through the Porhoët, the Rohan are related to the Dukes of Brittany, with whom the family intermingled again after its inception. They developed ties with the French and English royal houses as well, and played an important role in France and European history.
Alain I de Rohan, son of the viscount of Porhoët, was the first to take on the name of Rohan, after the place where he was born.
The main branch of the family went extinct when Jean II died childless in 1638; his title and possessions passed on to the cadet branch of Rohan-Gié.
When the first duke of Rohan, Henri II de Rohan-Gié, died, his title and name passed on to the Chabot family as his only daughter married Henri Chabot. This created the Rohan-Chabot lineage, which was not really a branch of the Rohan family.