Lothar (II) (born c. 835—died Aug. 8, 869, Piacenza, Italy) was a Frankish king of the area known as Lotharingia. His attempts to have his marriage dissolved so that he could marry his mistress caused much controversy and led to a bitter struggle between himself and Pope Nicholas I.
Lothar was the second son of the Frankish emperor Lothar I, ruler of the middle portion of the former empire of Charlemagne. Upon the death of Lothar I in 855, his realm was divided among his three sons, young Lothar receiving the area west of the Rhine from the North Sea to the Alps, which became known as Lotharingia (Lotharii regnum, or Lothar’s kingdom, the modern Lorraine). When his younger brother, Charles of Provence, died in 863, Charles’s kingdom was divided between the two surviving brothers: Louis II took Provence proper, and Lothar received the area around Vienne and Lyon.
In 855 Lothar had been forced by his father to marry Theutberga, a sister of Hicbert, the lay abbot of St. Maurice. Theutberga, however, remained childless, and from 857 the king tried to have the marriage dissolved and to take his mistress Waldrada, by whom he had had children, as his legitimate wife and queen. He accused his wife of incest with her brother, but her champion prevailed in the ordeal by boiling water, and Lothar was forced to take her back.
Lothar then induced two subservient archbishops, Günther of Cologne and Theutgaud of Trier, to start ecclesiastical proceedings against his wife. Two synods at Aachen dissolved the marriage and in 862 gave Lothar permission to marry Waldrada. He obtained the papal legate’s confirmation of this decision, probably through bribery, at a synod at Metz (June 863). Pope Nicholas I, however, reversed these decisions and took the unprecedented step of deposing archbishops Günther and Theutgaud (October 863). In August 865 another papal legate forced Lothar to take Theutberga back again.
In 867 Pope Nicholas I was succeeded by the more pliable Adrian II, and Lothar forced Theutberga to ask the new pope for a divorce herself. Lothar was received by the pope in 869 and was promised that the question would be considered at a council. He died shortly thereafter, while on his way home.