Sositheus of Alexandria Troas was a Greek tragic poet of the 3rd century BCE. His epithet denotes his origin from Alexandria in the Troad, not Egypt. He was a member of the Alexandrian Pleiad, the group of seven poets patronized by Ptolemy II Philadelphus at the Ptolemaic court. No further biographical details survive.

His most famous work was the satyr-play Daphnis or The Lityerses. He is also credited with a tragedy titled Aethlius. While the Byzantine Suda claims he wrote seven tragedies, his output is otherwise known only through fragments preserved in later quotations.

Sositheus is significant as a documented member of the Alexandrian Pleiad, exemplifying the Hellenistic revival of tragedy under royal patronage. His fragments, particularly from Lityerses, provide valuable, if limited, insight into the themes and style of a period from which almost no complete dramatic texts remain.