Giovanni Giustiniani Longo (Greek: Ιωάννης Λόγγος Ιουστινιάνης; Latin: Ioannes Iustinianus Longus) (died 1453) was a young Genoese captain, a member of one of the greatest families of the Republic, a kinsman to the powerful house of Doria in Genoa., and protostrator of the Eastern Roman (Byzantine) Empire. He led 700 professional soldiers, both Genovese and Greeks from the island of Chios, which at the time was part of the Republic of Genoa, to the defense of Constantinople against the Ottoman army of Sultan Mehmed II in 1453. (See also Fall of Constantinople.) He personally financed, organized and led this expedition on his own initiative, and upon arriving was placed in command of the land defenses by Emperor Constantine XI Palaiologos of the Byzantine Empire. Longo was key in controlling the land forces and keeping the Greeks, Genoese and Venetians from arguing with each other, and instead kept focused on repairing the land walls after the Ottoman cannon had shot holes in them. It was at least partly because of Longo's charisma that the Byzantine forces were able to hold out so long against overwhelming odds.
On May 29, 1453, during the final attack by Mehmet II, Longo was wounded by an Ottoman cannon while defending the walls of Constantinople. Some sources say the wound was caused by a crossbow bolt. Sources disagree about whether the wound was to his arm, leg, or chest, but it forced him to withdraw from his station at the land wall. He exited through the locked gate into the city, which opened up the opportunity for the fearful to flee, and panic spread throughout the lines.
Seeing the demoralization caused among the defenders by Longo's retreat, Sultan Mehmed II ordered a renewed assault that eventually defeated the Byzantines and Constantinople was taken by the Turks. Although Longo's men managed to escape with their general on board vessels fleeing Constantinople after its fall, Giovanni Giustiniani Longo died of the effects of his wound in the early days of June 1453.
Branches and descendants of Giovanni Giustiniani Longo's family still exist today in Greece and Italy.
- Commander Giustiniani was played by Kemal Ergünenç in Turkish film İstanbul'un Fethi (1951)
- Cengiz Coşkun plays Knight Giustiniani in Turkish film Fetih 1453 (2012). In the film, he is killed by Ulubatlı Hasan (İbrahim Çelikkol) during the last day in the siege of Constantinople.
- He appears as a major character in Mika Waltari's novel The Dark Angel