Giovanni Giustiniani Longo (Greek: Ιωάννης Λόγγος Ιουστινιάνης, Iōánnēs Lóggos Ioustiniánēs; Latin: Ioannes Iustinianus Longus; 1418–1453) 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.