Melissus of Samos was a Greek philosopher and military commander of the 5th century BCE. He is considered a member of the Eleatic school of philosophy founded by Parmenides. Ancient sources, including Plutarch, also record that he served as a Samian naval commander and won a significant battle against Athens around 441 BCE.

His philosophical contributions are known from a single, lost prose work, commonly titled On Nature or On What Is. The text survives only in fragments preserved by later authors. In it, Melissus systematically defended the core Eleatic doctrine that true reality is one, unchanging, and indivisible. He extended these ideas by arguing that this singular reality must also be infinite in both time and space, a point of divergence from Parmenides.

Melissus's significance lies in his rigorous logical development of Eleatic monism. His clear, deductive arguments rendered the school's challenging ideas more formidable and accessible. Scholars suggest 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 this way, Melissus played a key role in shaping the course of early Greek metaphysics.