Cratylus was a Greek philosopher from Athens who lived and taught in the late 5th and early 4th centuries BCE. He is known as a follower of Heraclitus, the earlier philosopher who famously argued that everything is in a state of constant change or "flux." According to ancient accounts, Cratylus was also an early teacher of the philosopher Plato before Plato studied with Socrates.

Cratylus is significant for pushing Heraclitus's ideas to an extreme conclusion. He is reported to have argued that if everything is always changing, then one cannot make any true statement about the world. He reportedly believed that one could not even step into the same river once, because it changes in the very moment of stepping. This radical view led him to a deep skepticism about language. Some ancient sources claim he eventually stopped speaking altogether, only pointing at things with his finger to avoid making false statements.

No written works by Cratylus survive. Our knowledge of his ideas comes entirely from reports by other philosophers, primarily Aristotle. Modern scholars see Cratylus as an important philosophical link. His extreme interpretation of Heraclitean flux is considered a direct influence on Plato, who later developed his theory of unchanging "Forms" as a response to the problem of a constantly changing world.