Giovanni Giustiniani Longo (, "Iōánnēs Lóngos Ioustiniánēs"; ; 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.