Flavius Julius Crispus (; c. 303 – 326) was the eldest son of the Roman emperor Constantine the Great and his junior emperor (caesar) from March 317 until his execution by his father in 326. The grandson of the augustus Constantius I, Crispus was the elder half-brother of the future augustus Constantine II and became co-caesar with him and with his cousin Licinius II at Serdica, part of the settlement ending the Cibalensean War between Constantine and his father's rival Licinius I. Crispus ruled from Augusta Treverorum (Trier) in Roman Gaul between 318 and 323 and defeated the navy of Licinius I at the Battle of the Hellespont in 324, which with the land Battle of Chrysopolis won by Constantine forced the resignation of Licinius and his son, leaving Constantine the sole augustus and the Constantinian dynasty in control of the entire empire. It is unclear what was legal status of the relationship Crispus's mother Minervina had with Constantine; Crispus may have been an illegitimate son.

Crispus's tutor in rhetoric was the Late Latin historian of Early Christianity, Lactantius. Crispus may be the young prince depicted on the Gemma Constantiniana, a great cameo depicting Constantine and his wife Fausta, though the depiction may instead be of Fausta's own son, the future augustus Constantius II. While at Augusta Treverorum, Crispus's praetorian prefect for the prefecture of Gaul was the great Junius Annius Bassus. After his elevation to imperial rank, at which point he was also entitled princeps iuventutis ("Prince of Youth"), the Latin rhetorician Nazarius composed a panegyric preserved in the Panegyrici Latini, which honoured Crispus's military victories over the Franks in c. 319. Crispus was three times Roman consul, for the years 318, 321, and 324.

Within two years of the defeat and surrender of Licinius, Constantine had not only put his brother-in-law and former co-augustus to death, but also executed his nephew Licinius II, the son of his sister Flavia Julia Constantia. According to the Latin histories of Ammianus Marcellinus and Aurelius Victor, after a trial whose real circumstances are mysterious, Constantine executed Crispus at Pola (Pula) in 326. Fausta, whose son Constantius II became caesar in November 324, was also put to death, and the Late Greek historian Zosimus and the Byzantine Greek writer Joannes Zonaras wrote that Constantine had accused Crispus of incest with his stepmother. After his death, Crispus was subjected to damnatio memoriae.