Lambert III was the pretender to the County of Nantes between 851 and his death in 862.
Lambert was the son of Lambert II of Nantes, at whose death in 851, the administration of the region fell effectively to the Bretons. His mother was Tetrata of Lombardy, a daughter of the eldest son of Charlemagne, Pepin of Italy. Lambert III married Rotrude of Italy, a daughter of Holy Roman Emperor Lothair I. The Emperor was a first cousin of Lambert's mother, as both were grandchildren of Charlemagne.