Justin (Latin Marcus Junianius (or Junianus) Justinus) was a Latin historian who lived under the Roman Empire. His name is mentioned only in the title of his own history, and there it is in the genitive, which would be M. Juniani Justini no matter which nomen he bore.
Of his personal history nothing is known. He is the author of Historiarum Philippicarum libri XLIV, a work described by himself in his preface as a collection of the most important and interesting passages from the voluminous Historiae philippicae et totius mundi origines et terrae situs, written in the time of Augustus by Pompeius Trogus.
His date is uncertain, except that he must have lived after Trogus. He writes that the Romans and the Parthians have divided the world between them; while this is presumably from Trogus, it would be an anachronism after the rise of the Sassanian Empire in the 3rd century AD. Although Latin changed slowly, Justin's language would also be consistent with a date in the 2nd century AD. Ronald Syme argues for a date around 390, immediately before the compilation of the Augustan History, and dismisses the anachronism as unimportant; readers would understand that these passages represented Trogus' time, not their own.
The work of Trogus is lost; but the prologi or arguments of the text are preserved by Pliny and other writers. Although the main theme of Trogus was the rise and history of the Macedonian monarchy, Justin yet permitted himself considerable freedom of digression, and thus produced an idiosyncratic anthology instead of a mundane summary (or 'epitome') of the work.
The Eleventh Edition, Encyclopædia Britannica, concluded that his history contained much valuable information and that the style, though far from perfect, is clear and occasionally elegant. The book was much used in the Middle Ages, when the author was sometimes confused with Justin Martyr.