In Greek mythology and later Roman mythology, the Cyclopes (/saɪˈkloʊpiːz/ sy-KLOH-peez; Greek: Κύκλωπες, Kýklōpes, "Circle-eyes" or "Round-eyes"; singular Cyclops /ˈsaɪklɒps/ SY-klops; Κύκλωψ, Kýklōps) are giant one-eyed creatures. Three groups of Cyclopes can be distinguished. In Hesiod's Theogony, the Cyclopes are the three brothers, Brontes, Steropes, and Arges, who made Zeus's weapon, the thunderbolt. In Homer's Odyssey, they are an uncivilized group of shepherds, the brethren of Polyphemus encountered by Odysseus. A third group of Cyclopes reputedly built the Cyclopean walls of Mycenae and Tiryns.

In Cyclops, the fifth-century BC play by Euripides, a chorus of satyrs offers comic relief based on the encounter of Odysseus and Polyphemus. The third-century BC poet Callimachus makes the Hesiodic Cyclopes the assistants of smith-god Hephaestus, as does Virgil in the Latin epic Aeneid, where he seems to equate the Hesiodic and Homeric Cyclopes.

From at least the fifth century BC, Cyclopes have been associated with the island of Sicily and the volcanic Aeolian Islands.

Three groups of Cyclopes can be distinguished: the Hesiodic, the Homeric and the wall-builders. In Hesiod's Theogony, the Cyclopes are the three brothers: Brontes, Steropes, and Arges, sons of Uranus and Gaia, who made for Zeus his characteristic weapon, the thunderbolt. In Homer's Odyssey, the Cyclopes are an uncivilized group of shepherds, one of whom, Polyphemus, the son of Poseidon, is encountered by Odysseus. Cyclopes were also said to have been the builders of the Cyclopean walls of Mycenae and Tiryns. A scholiast, quoting the fifth-century BC historian Hellanicus, tells us that, in addition to the Hesiodic Cyclopes (whom the scholiast describes as "the gods themselves"), and the Homeric Cyclopes, there was a third group of Cyclopes: the builders of the walls of Mycenae.

Hesiod, in the Theogony (c. 700 BC), described three Cyclopes: Brontes, Steropes, and Arges, who were the sons of Uranus (Sky) and Gaia (Earth), and the brothers of the Titans and Hundred-Handers, and who had a single eye set in the middle of their foreheads. They made for Zeus his all-powerful thunderbolt, and in so doing, the Cyclopes played a key role in the Greek succession myth, which told how the Titan Cronus overthrew his father Uranus, and how in turn Zeus overthrew Cronus and his fellow Titans, and how Zeus was eventually established as the final and permanent ruler of the cosmos. The names that Hesiod gives them, Arges (Bright), Brontes (Thunder), and Steropes (Lightning), reflect their fundamental role as thunderbolt makers. As early as the late seventh century BC, the Cyclopes could be used by the Spartan poet Tyrtaeus to epitomize extraordinary size and strength.

According to the accounts of Hesiod and the mythographer Apollodorus, the Cyclopes had been imprisoned by their father Uranus. Zeus later freed the Cyclopes, and they repaid him by giving him the thunderbolt. The Cyclopes provided for Hesiod, and other theogony-writers, a convenient source of heavenly weaponry, since the smith-god Hephaestus—who would eventually take over that role—had not yet been born. According to Apollodorus, the Cyclopes also provided Poseidon with his trident and Hades with his cap of invisibility, and the gods used these weapons to defeat the Titans.

Although the primordial Cyclopes of the Theogony were presumably immortal (as were their brothers the Titans), the sixth-century BC Hesiodic Catalogue of Women, has them being killed by Apollo. Later sources tell us why: Apollo's son Asclepius had been killed by Zeus' thunderbolt, and Apollo killed the Cyclopes, the makers of the thunderbolt, in revenge. According to a scholiast on Euripides' Alcestis, the fifth-century BC mythographer Pherecydes supplied the same motive, but said that Apollo, rather than killing the Cyclopes, killed their sons (one of whom he named Aortes) instead. No other source mentions any offspring of the Cyclopes. A Pindar fragment suggests that Zeus himself killed the Cyclopes to prevent them from making thunderbolts for anyone else.

The Cyclopes' prowess as craftsmen is stressed by Hesiod who says "strength and force and contrivances were in their works." Being such skilled craftsmen of great size and strength, later poets, beginning with the third-century BC poet Callimachus, imagine these Cyclopes, the primordial makers of Zeus' thunderbolt, becoming the assistants of the smith-god Hephaestus, at his forge in Sicily, underneath Mount Etna, or perhaps the nearby Aeolian Islands. In his Hymn to Artemis, Callimachus has the Cyclopes on the Aeolian island of Lipari, working "at the anvils of Hephaestus", make the bows and arrows used by Apollo and Artemis. The first-century BC Latin poet Virgil, in his epic Aeneid, has the Cyclopes: "Brontes and Steropes and bare-limbed Pyracmon" toil under the direction of Vulcan (Hephaestus), in caves underneath Mount Etna and the Aeolian islands. Virgil describes the Cyclopes, in Vulcan's smithy forging iron, making a thunderbolt, a chariot for Mars, and Pallas's Aegis, with Vulcan interrupting their work to command the Cyclopes to fashion arms for Aeneas. The later Latin poet Ovid also has the Hesiodic Cyclopes, Brontes and Steropes (along with a third Cyclops named Acmonides), work at forges in Sicilian caves.