Xuthus was a Pythagorean philosopher from the ancient Greek city of Rhegium in southern Italy. His exact dates are uncertain, but he likely lived during the 5th or 4th century BCE. He is known only from a few later ancient sources, which identify him as the father of the poet Telestes.

No writings by Xuthus survive, and none are recorded by title. He is remembered solely for a single theory about the natural world, as reported by the philosopher Aristotle. According to this report, Xuthus proposed that animals were generated from a slime or foam produced when rainwater mixed with earth. The Neoplatonist commentator Simplicius later explicitly identified this Xuthus as a Pythagorean from Rhegium.

Modern scholars regard Xuthus as a very minor figure in early Greek philosophy. His significance lies only in his inclusion in ancient lists of Pythagoreans and as an example of early speculative theories on the origin of life. His mention by Aristotle preserves his name in the history of Presocratic thought.