Giovanni Giustiniani Longo (Greek: Ιωάννης Λόνγος Ιουστινιάνης, Iōánnēs Lóngos Ioustiniánēs; Latin: Ioannes Iustinianus Longus; 1418 – 1 June 1453) was a Genoese captain, a member of one of the greatest families of the Republic of Genoa, a kinsman to the powerful house of Doria in Genoa, and protostrator of the Eastern Roman (Byzantine) Empire. He led 700 professional soldiers, both Genoese 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. 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. Giustiniani was key in controlling the land forces and keeping the Greeks, Genoese and Venetians from quarrelling with each other, and instead kept focused on repairing the land walls after the Ottoman cannons had shot holes in them. It was at least partly because of Giustiniani's charisma that the Byzantine forces were able to hold out so long against overwhelming odds.
On 29 May 1453, during the final attack by Mehmed II, Giustiniani 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 loss of morale caused among the defenders by Giustiniani's retreat, Mehmed II ordered a renewed assault that eventually overwhelmed the Byzantine defenses and Constantinople was taken by the Turks. Although Giustiniani's men managed to escape with their general after its fall, Giustiniani died from his wounds on 1 June 1453.
His body was carried by his comrades to the then Genoese island of Chios. His tomb in the Church of San Domenico on Chios is lost (possibly from the earthquake of 1881), but several descriptions survive.