Melissus of Samos was a Greek philosopher and military commander who lived during the 5th century BCE. He is identified as a member of the Eleatic school of philosophy, which was founded by Parmenides. Ancient historical accounts, such as those by Plutarch, also report that Melissus served as a naval commander for Samos and won a notable battle against Athens around 441 BCE.

His philosophical contributions are contained in a single, lost prose work, often titled On Nature or On What Is. The text survives only in fragments preserved by later writers. In it, Melissus systematically defended the core Eleatic belief that true reality is one, unchanging, and indivisible. According to modern scholars, he extended these ideas by arguing that this singular reality must also be infinite in both time and space, a point on which he differed from his predecessor Parmenides.

Melissus's significance lies in his rigorous logical development of Eleatic monism. His clear, deductive arguments made the school's challenging ideas more formidable and accessible. Academics suggest that his work directly influenced later philosophical debates, particularly by provoking responses from atomist philosophers like Leucippus and Democritus, who sought to explain the apparent existence of motion and plurality in the world. Thus, Melissus played a key role in shaping the course of early Greek metaphysics.