Posidonius of Olbia was a minor Greek historian of the 1st century BCE, a native of the Pontic colony of Olbia. He is distinguished from the Stoic philosopher Posidonius of Apamea. Beyond this, no biographical details are recorded.

His sole known work is a lost local history, On the Customs of the Sauromatians, which survives only in fragments. These are preserved primarily through quotations in Strabo’s Geographica.

Posidonius is significant as a source of ethnographic information on the peoples north of the Black Sea, particularly the Sarmatians. His work, utilized by Strabo, contributes to the Greco-Roman understanding of the Pontic steppe from the perspective of a Greek colony on the periphery of the Hellenistic world.