Justus was born and raised in Rome. He was the second son born to the Roman Jewish historian Josephus and his fourth wife, a distinguished unnamed Greek Jewish noblewoman from Crete. The parents of Justus met in Rome, and Josephus describes his mother, as one ‘in character who excelled many women, as her subsequent life demonstrated’. Justus had an older paternal half-brother, Hyrcanus, and a younger brother, Flavius Simonides Agrippa. His paternal uncle was Matthias, while his paternal grandparents were Matthias and his wife, an unnamed Jewish noblewoman. His paternal grandmother was an aristocratic woman who descended from royalty and of the former ruling Hasmonean Dynasty. His paternal grandfather descended from the priestly order of the Jehoiarib, which was the first of the twenty four-orders of priests in the Temple in Jerusalem and was a descendant of the High Priest Jonathon. Jonathon may have been Alexander Jannaeus, the High Priest and Hasmonean ruler who governed Judea from 103 BC-76 BC.
Justus’ cognomen was a very common Latin name in the Jewish Catacombs of Rome. Josephus gave his son a Latin name in preference to a Greek one, despite the fact that Greek was spoken more among Roman Jews than Latin. Justus was a contemporary of the ruling Flavian dynasty and Nerva–Antonine dynasty of the Roman Empire. He was alive when Josephus was compiling his historical writings and when his father died about 100. Unfortunately, little is known on his remaining life.
- Fergus, The History of the Jewish People in the Age of Jesus Christ (175 B.C. - A.D. 135), pp. 45-6
- Josephus, Flavius Josephus: translation and commentary pp. 10 and 170
- Nodet, A search for the origins of Judaism: from Joshua to the Mishnah, p. 250
- Josephus, Flavius Josephus: translation and commentary, p. 10
- M. Fergus, S. Emil & V. Geza, The History of the Jewish People in the Age of Jesus Christ (175 B.C. - A.D. 135), Continuum International Publishing Group, 1973
- É. Nodet, A search for the origins of Judaism: from Joshua to the Mishnah, Continuum International Publishing Group, 1997
- F. Josephus & S. Mason, Flavius Josephus: translation and commentary, BRILL, 2001