Joseph Loth (27 December 1847 – 1 April 1934) was a French linguist and historian who specialised in the study of Celtic languages.

At this time, he made the acquaintance of Henri d'Arbois de Jubainville, who encouraged him to study Celtic languages.

That same year he founded the journal Annales de Bretagne, in which he published numerous studies and edited until 1910.

He was appointed professor at the Collège de France in 1910 and was elected member of the Académie des Inscriptions et Belles-Lettres in 1919.

Loth published numerous articles in the Revue Celtique, re-edited Pierre de Chalons' Breton-French dictionary and translated into French important Celtic literature including the Mabinogion.