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Sallustius or Sallust (; Ancient Greek: Σαλούστιος Saloustios) was a 4th-century writer, a friend of the Roman Emperor Julian. He wrote the treatise On the Gods and the Cosmos, a kind of catechism of 4th-century Hellenic paganism.

Sallustius' work owes much to that of Iamblichus of Chalcis, who synthesized Platonism with Pythagoreanism and theurgy, and also to Julian's own philosophical writings. The treatise is quite concise, and generally free of the lengthy metaphysical theorizing of the more detailed Neoplatonic texts. Its aim is in part "to parry the usual onslaughts of Christian polemic" in the face of Christianity's growing preeminence, and "me[e]t theology with theology".

Sallustius' exact identity is a matter of some uncertainty. By some he is identified as Flavius Sallustius who was praetorian prefect of Gaul from 361 until 363 and Julian's colleague as consul in 363), by others with Saturninius Secundus Salutius a native of Gaul who was praetorian prefect of the Orient in 361).