In Greek mythology, Patroclus (generally pronounced /pəˈtroʊkləs/; Ancient Greek: Πάτροκλος, romanized: Pátroklos, lit. 'glory of the father') was a Greek hero of the Trojan War and an important character in Homer's Iliad. Born in Opus, Patroclus was the son of the Argonaut Menoetius. When he was a child, he was exiled from his hometown and was adopted by Peleus, king of Phthia. There, he was raised alongside Peleus's son, Achilles, a childhood friend, who became a close wartime companion and possible lover. When the tide of the war turned against the Achaeans, Patroclus, disguised as Achilles and defying his orders to retreat in time, led the Myrmidons in battle against the Trojans and was eventually killed by the Trojan prince, Hector. Enraged by Patroclus's death, Achilles ended his refusal to fight, resulting in significant Greek victories.

The Latinized name Patroclus derives from the Ancient Greek Pátroklos (Πάτροκλος), meaning "glory of his father," from πατήρ (patḗr, "father" stem pátr-) and κλέος (kléos, "glory"). A variation of the name with the same components in different order is Kleópatros, while the feminine form of the name is Cleopatra.

There are at least three pronunciations of the name 'Patroclus' in English. Because the penultimate syllable is light in Latin prose (pă′.trŏ.clŭs), the antepenult was stressed in Latin and would normally be stressed in English as well, for /ˈpæt.rə.kləs/ (analogous to 'Sophocles'). However, this pronunciation is seldom encountered: for metrical convenience, Alexander Pope had made the 'o' long, and thus stressed, in his translation of Homer, following a convention of Greek and Latin verse, and that pronunciation – of Latin pa.trō′.clus – has stuck, for English /pəˈtroʊ.kləs/. Moreover, because in prose, a penultimate Greco-Latin short o (omicron) would only be stressed in a closed syllable, the penult has sometimes been misanalysed as being closed (*pă.trŏc′.lŭs), which would change the English o to a short vowel: /pəˈtrɒk.ləs/.

In the account of Dares the Phrygian, Patroclus was illustrated as "... handsome and powerfully built. His eyes were gray. He was modest, dependable, wise, a man richly endowed."

Patroclus was the son of Menoetius (hence called Menoetiades Μενοιτιάδης, meaning "son of Menoetius") by either Philomela or Polymele, Sthenele, Periopis, or lastly Damocrateia. His only sibling was Myrto, mother of Eucleia by Heracles. Homer also references Menoetius as the individual who gave Patroclus to Peleus. Menoetius was the son of Actor, king of Opus in Locris, by Aegina, daughter of Asopus. Patroclus was Achilles's first cousin once removed through their paternal family connection to Aegina, as Achilles was the son of Peleus and grandson of Aeacus, son of Aegina by Zeus.

During his childhood, Patroclus had accidentally killed his playmate Clysonymus over a game of dice. As a result, he was exiled from his home, Opus, with Menoetius sending him to Peleus, king of Phthia and father of Achilles. Peleus named Patroclus Achilles's "squire", as they both grew up together and became close friends. Patroclus acted as a male role model for Achilles, being both kinder than him as well as wiser regarding counsel. Patroclus's early life, including his flight to the house of Peleus, is narrated later in the Iliad, when his ghost appears to Achilles reminding him about his past and giving him advice about his burial.

According to Photius, Ptolemy Hephaestion (probably referring to Ptolemy Chennus) wrote that Patroclus was also loved by the sea god Poseidon, who taught him the art of riding horses.

Local tradition in Laconia held that Achilles killed Las when he came to their land to seek the hand of Helen, daughter of Tyndareus. However, the geographer Pausanias disagrees, stating that it was actually Patroclus who killed Las, arguing that Patroclus, not Achilles, was one of Helen's suitors.