Possis of Magnesia was a Greek historian who lived during the 3rd century BCE. He came from the city of Magnesia, likely the one located on the Maeander River in Asia Minor. No details about his personal life are known. He is remembered for writing a single major work, The Deeds of Alexander, a history of Alexander the Great.

This history has not survived to the present day and is known only through a handful of brief fragments and references in later authors. The Roman writer Plutarch quotes from Possis’s work twice in his own biography of Alexander, citing an anecdote about Alexander’s drinking and a description of a painting of the king. These citations suggest that Possis’s account included colorful biographical stories and details.

According to modern scholars, Possis is considered a minor figure among the many historians who wrote about Alexander. His significance lies in being one of the lost sources that later historians used. Academics often group him with other writers of the so-called "vulgate" tradition, whose popular and dramatic narratives helped shape the legendary image of Alexander that was passed down through history.