Théodore Claude Henri, vicomte Hersart de la Villemarqué (6 July 1815 – 8 December 1895) was a Breton philologist and man of letters.

He was descended from an old Breton family, which counted among its members an Hersart who had followed Saint Louis to the Crusades, and another who was a companion in arms of Bertrand du Guesclin.

[1] His works include: Contes populaires des anciens Bretons (1842), to which was prefixed an essay on the origin of the romances of the Round Table; Essai sur l'histoire de la langue bretonne (1837); Poèmes des bardes bretons du sixième siècle (1850); La Légende celtique en Irlande, en Cambrie et en Bretagne (1859).

The popular Breton songs published by him in 1839 as Barzaz Breiz were considerably retouched.

[1] On the subject of the doubtful authenticity of Barzaz Breiz, see Luzel's Preface to his Chansons populaires de la Basse-Bretagne, and, for a list of works on the subject, the Revue Celtique (vol.