This article is about a hero of the Trojan War, king Diomedes of Argo, one of the principal generals on the Greek side. The Mares of Diomedes were stolen by a hero of Argo (Herakles), from a different Diomedes, who was king of Thrace. The Diomede Islands have a different article.

In Greek mythology, Diomêdês ("god-like cunning") was the son of Tydeus and Deipyle and a favored hero of Athena. He was one of the Epigonoi and later became King of Argos, succeeding his grandfather, Adrastus.

In the Iliad, Diomedes is one of the most attractive figures among the Greek generals. Along with Sthenelus, he leads the Argive armies. He has a horse named Lampos. One of his companions, a brother-in-arms, is named Euryalos.

Book V of the Iliad centers around the battlefield valor of Diomedes. He duels Aeneas and nearly kills him, but Aphrodite, Aeneas' mother, comes to his aid and tries to cover him up. Diomedes wounds Aphrodite and she drops her son, fleeing to Mount Olympus. Aeneas is then enveloped in a cloud by Apollo, who takes him to Pergamos, a sacred spot in Troy. Artemis heals Aeneas there.

Later in the same melee, Diomedes fights with Hector and sees Ares, the war-god, fighting on the Trojans' side. Diomedes calls for his soldiers to fall back slowly. Hera, Ares' mother, sees Ares' interference and asks Zeus, Ares' father, for permission to drive Ares away from the battlefield. Hera encourages Diomedes to attack Ares and he threw his spear at the god. Athena drives the spear into Ares' belly: bellowing in pain, the wounded god ascends to Olympus in a column of smoke, forcing the Trojans to fall back.

Subsequently, in a night raid on the Trojan camp, Diomedes and Odysseus steal King Rhesus's team of fine horses. This does not have an impact on the outcome of the battle, but it demonstrates the two kings' courage and guile.

Like the other major Greek characters, Diomedes is alive and well as the Iliad comes to a close. In the Odyssey the reader is told that when the Trojan War was over, Diomedes returned home. Other stories of his exploits come from later classical sources.

Diomedes and Odysseus stole the Palladium and took it to Argos. Diomedes also killed Merops' two sons.

After his return home, his wife, Aegiale had been unfaithful to him and Diomedes left for Italy, where he founded the cities of Brindisium and Arpus Hippium .

At some point, Diomedes restored Oeneus to the throne of Calydon after his brother's (Agrius) sons had overthrown him.

In the Divine Comedy, Dante sees Diomedes in the eighth circle of Hell, where he is condemned together with Ulysses to be imprisoned for eternity in a sheet of flame. The specific sin which Dante has in mind as to Diomedes appears to be the theft of the Palladium.