Constantine IX Monomachus (c. 1000 - January 11, 1055) reigned as Byzantine emperor from June 11, 1042 to January 11, 1055. He had been chosen by Zoë as a husband and co-emperor in 1042, although he had been exiled for conspiring against her previous husband Michael IV. They ruled together until Zoë died in 1050.

In 1043 he relieved General George Maniaces from his command in Italy, and Maniaces declared himself emperor. His troops were about to defeat Constantine in battle, but he was wounded and died on the field, ending the crisis. Immediately after the victory, Constantine was attacked by a fleet from the Kievan Rus', which had probably been hired by Maniaces. They too were defeated, with the help of Greek fire.

In 1046 the Byzantines came into contact for the first time with the Seljuks. They met in battle in Armenia in 1048, and settled a truce the following year. However, Constantine was forced to disband the Armenian troops for financial reasons in 1053, leaving the eastern frontier poorly defended.

In 1054 the centuries-old differences between the Greek and Roman churches led to their final separation. Legates from Pope Leo IX excommunicated the Patriarch of Constantinople Michael Cerularius when Cerularius would not agree to adopt western church practises, and in return Cerularius excommunicated the legates. This annulled Constantine's attempts to ally with the Pope against the Normans.

Constantine tried to intervene, but he fell ill and died on January 11 of the following year. Theodora, the elderly daughter of Constantine VIII who had ruled with her sister Zoë, was recalled and named empress.

Constantine was also a patron of the scholar Michael Psellus the Younger, whose Chronographica records the history of Constantine's reign.

Constantine's nickname, Monomachos (one who fights his own battles) was inherited by his grandson, Vladimir Monomakh.