In Greek mythology, Achilles (/əˈkɪliːz/ ^ⓘ ə-KIL-eez) or Achilleus (Ancient Greek: Ἀχιλλεύς, romanized: Achilleús) was a hero of the Trojan War who was known as being the greatest of all the Greek warriors. The central character in Homer's Iliad, he was the son of the Nereid Thetis and Peleus, king of Phthia and famous Argonaut. Achilles was raised in Phthia along with his childhood companion Patroclus and received his education by the centaur Chiron. In the Iliad, he is presented as the commander of the mythical tribe of the Myrmidons.

Achilles's most notable feat during the Trojan War was the slaying of the Trojan prince Hector outside the gates of Troy. Although the death of Achilles is not presented in the Iliad, other sources concur that he was killed near the end of the Trojan War by Paris, who shot him with an arrow. Later legends (beginning with Statius's unfinished epic Achilleid, written in the first century CE) state that Achilles was invulnerable in all of his body except for one heel. According to that myth, when his mother Thetis dipped him in the river Styx as an infant, she held him by one of his heels, leaving it untouched by the waters and thus his only vulnerable body part.

Alluding to these legends, the term Achilles' heel has come to mean a point of weakness which can lead to downfall, especially in someone or something with an otherwise strong constitution. The Achilles tendon is named after him following the same legend.

Linear B tablets attest to the personal name Achilleus in the forms a-ki-re-u and a-ki-re-we, the latter being the dative of the former. The name grew more popular, becoming common soon after the seventh century BCE and was also turned into the female form Ἀχιλλεία (Achilleía), attested in Attica in the fourth century BCE (IG II² 1617) and, in the form Achillia, on a stele in Halicarnassus as the name of a female gladiator fighting an "Amazon".

Achilles's name can be analyzed as a combination of ἄχος (áchos), 'distress, pain, sorrow, grief', and λαός (laós), 'people, soldiers, nation', resulting in a proto-form *Akhí-lāu̯os, 'he who has the people distressed' or 'he whose people have distress'. The grief or distress of the people is a theme raised numerous times in the Iliad (and frequently by Achilles himself). Achilles's role as the hero of grief or distress forms an ironic juxtaposition with the conventional view of him as the hero of κλέος kléos ('glory', usually in war). Furthermore, laós has been construed by Gregory Nagy, following Leonard Palmer, to mean 'a corps of soldiers', a muster. With this derivation, the name obtains a double meaning in the poem: when the hero is functioning rightly, his men bring distress to the enemy, but when wrongly, his men get the grief of war. The poem is in part about the misdirection of anger on the part of leadership.

Some researchers deem the name a loan word, possibly from a Pre-Greek language. Achilles's descent from the Nereid Thetis and a similarity of his name with those of river deities such as Acheron and Achelous have led to speculations about his being an old water divinity . Robert S. P. Beekes has suggested a Pre-Greek origin of the name, based among other things on the coexistence of -λλ- and -λ- in epic language, which may account for a palatalized phoneme /l^y/ in the original language.

Among the appellations under which Achilles is generally known are the following:

Achilles was the son of Thetis—a Nereid and daughter of the Old Man of the Sea—and Peleus, the king of the Myrmidons. Zeus and Poseidon had been rivals for Thetis's hand in marriage until Prometheus, the fore-thinker, warned Zeus of a prophecy (originally uttered by Themis, goddess of divine law) that Thetis would bear a son greater than his father. For this reason, the two gods withdrew their pursuit, and had her wed Peleus.