Idaeus of Himera was a philosopher from the 5th century BCE, during the Classical period of ancient Greece. He lived in Himera, a Greek colony on the island of Sicily. According to the much later historian Damascius, Idaeus was a follower of Pythagoreanism, a philosophical and religious school that was influential in southern Italy and Sicily at the time.

No writings by Idaeus survive. He is known only from a single report by Damascius, which states that Idaeus, along with another thinker, taught that the fundamental principles of the universe were a pair of opposites: the Limited and the Unlimited. This idea is a core part of early Pythagorean cosmology, which tried to explain reality through such opposing principles.

His historical importance is minimal but specific. He is a documented example of how Pythagorean ideas spread through the Greek colonies of the western Mediterranean. His near-total obscurity is also typical, illustrating how our knowledge of many early Greek thinkers depends entirely on brief mentions by later authors who compiled histories of philosophy.