Aristobulus of Cassandreia was a Greek historian of the 4th century BCE. He participated in the campaigns of Alexander the Great, though his precise role—whether as an architect, engineer, or soldier—remains unclear. A contemporary of Alexander, he began composing his history at an advanced age, reportedly after 84. His city of origin, Cassandreia, was a Macedonian city originally named Potidaea and later refounded by Cassander.

His only known work is the History of Alexander, a detailed account of Alexander's campaigns written late in his life. The text does not survive intact but is preserved in fragments and through extensive quotation by later historians like Arrian. Aristobulus is a significant primary source for Alexander's reign. Alongside Ptolemy I Soter, he was one of Arrian's two main sources for the Anabasis of Alexander; Arrian praised him for his reliability and accuracy.

His work exhibits a rationalizing tendency, offering naturalistic explanations for events often considered miraculous. Later historians like Strabo and Plutarch also used his history, which provides valuable participant perspectives on geography, events, and Alexander's character.