Diogenes of Apollonia was a philosopher of the 5th century BCE, often considered the last of the Pre-Socratic thinkers. His exact origins are uncertain, as several ancient cities were named Apollonia, but he was likely active around 440–430 BCE and may have spent time in Athens. He is distinct from the later philosopher Diogenes the Cynic.

He was a monist, meaning he believed all things were made from a single fundamental substance. He revived and expanded upon the earlier theory of Anaximenes, arguing that this substance was Air. Diogenes proposed that this Air was intelligent and directed the order of the cosmos. He also applied this idea to explain human biology, suggesting that air (pneuma) within the body was responsible for life, sensation, and thought.

His writings are lost and survive only in fragments quoted by later authors. His major known work was On Nature. Other titles attributed to him include Against the Sophists and Meteorology, though these may have been parts of his larger work rather than separate books.

His historical importance lies in his synthesis of earlier ideas into a final, comprehensive monistic system. According to modern scholars, his work was an attempt to counter the pluralist theories of contemporaries like Empedocles and Anaxagoras by arguing for a single, intelligent governing principle. His ideas were notable enough in Athens to be satirized by the comic playwright Aristophanes in The Clouds.