Charles the Fat (in French: Charles le Gros) (c. 832–January 13, 888) was a King of France and, as Charles III, Holy Roman Emperor. He was the son of Louis the German. Granted lordship over Swabia in 876, he became King of Italy in 879 upon the abdication of his older brother Carloman. Crowned Emperor in 881, his succession to the Kingdom of Saxony the following year reunited the entire Kingdom of the East Franks (Germany). Upon the death of Carloman, the King of the West Franks (France), on December 12, 884, he achieved that throne as well, thus reviving, if only briefly, the entire Carolingian Empire, aside from Burgundy.

His rise to power was accompanied by hopes of a general revival in western Europe, but he proved unequal to the task. Lethargic and inept—he is known to have had repeated illnesses which are believed to have been epilepsy—he conducted several unsuccessful expeditions in Italy against Saracen incursions, and purchased peace with Viking raiders in Paris in 886.

Increasingly seen as spineless and incompetent, matters came to a head in late 887, when an ambitious nephew, Arnulf, fomented a general rebellion and seized Germany in November. Charles did nothing to prevent the move and, retiring to Neidingen , died two months later, on January 13, 888. His empire broke asunder, never to be restored—Arnulf retained Germany and Lotharingia, France was gained by Odo, Count of Paris, Italy by Berengar of Friuli, Upper Burgundy by Rudolph and Provence by Louis the Blind.