In ancient Greek religion and mythology, Helios (/ˈhiːliəs, -ɒs/; Ancient Greek: Ἥλιος pronounced [hɛ̌ːlios], lit. 'Sun'; Homeric Greek: Ἠέλιος) is the god who personifies the Sun. His name is also Latinized as Helius, and he is often given the epithets Hyperion ("the one above") and Phaethon ("the shining"). Helios is often depicted in art with a radiant crown and driving a horse-drawn chariot through the sky. He was a guardian of oaths and also the god of sight. Though Helios was a relatively minor deity in Classical Greece, his worship grew more prominent in late antiquity thanks to his identification with several major solar divinities of the Roman period, particularly Apollo and Sol. The Roman Emperor Julian made Helios the central divinity of his short-lived revival of traditional Roman religious practices in the 4th century AD.

Helios figures prominently in several works of Greek mythology, poetry, and literature, in which he is often described as the son of the Titans Hyperion and Theia and brother of the goddesses Selene (the Moon) and Eos (the Dawn). Helios's most notable role in Greek mythology is the story of his mortal son Phaethon. In the Homeric epics, his most notable role is the one he plays in the Odyssey, where Odysseus's men despite his warnings impiously kill and eat Helios's sacred cattle that the god kept at Thrinacia, his sacred island. Once informed of their misdeed, Helios, in wrath, asks Zeus to punish those who wronged him, and Zeus, agreeing, strikes their ship with a thunderbolt, killing everyone except Odysseus himself, the only one who had not harmed the cattle and was allowed to live.

Due to his position as the sun, he was believed to be an all-seeing witness and thus was often invoked in oaths. He also played a significant part in ancient magic and spells. In art he is usually depicted as a beardless youth in a chiton holding a whip and driving his quadriga, accompanied by various other celestial gods such as Selene, Eos, or the stars. In ancient times he was worshipped in several places of ancient Greece, though his major cult centres were the island of Rhodes, of which he was the patron god, Corinth, and the greater Corinthia region. The Colossus of Rhodes, a gigantic statue of the god, adorned the port of Rhodes until it was destroyed in an earthquake.

The Greek noun ἥλιος (GEN ἡλίου, DAT ἡλίῳ, ACC ἥλιον, VOC ἥλιε) (from earlier ἁϝέλιος /hāwelios/) is the inherited word for the Sun from Proto-Indo-European *seh₂u-el which is cognate with Latin sol, Sanskrit surya, Old English swegl, Old Norse sól, Welsh haul, Avestan hvar, etc. The Doric and Aeolic form of the name is Ἅλιος, Hálios. In Homeric Greek his name is spelled Ἠέλιος, Ēélios, with the Doric spelling of that being Ἀέλιος, Aélios. In Cretan it was Ἀβέλιος (Abélios) or Ἀϝέλιος (Awélios). The Greek view of gender was also present in their language. Ancient Greek had three genders (masculine, feminine and neuter), so when an object or a concept was personified as a deity, it inherited the gender of the relevant noun; helios is a masculine noun, so the god embodying it is also by necessity male. The female offspring of Helios were called Heliades, the male Heliadae.

The author of the Suda lexicon tried to etymologically connect ἥλιος to the word ἀολλίζεσθαι, aollízesthai, "coming together" during the daytime, or perhaps from ἀλεαίνειν, aleaínein, "warming". Plato in his dialogue Cratylus suggested several etymologies for the word, proposing among others a connection, via the Doric form of the word halios, to the words ἁλίζειν, halízein, meaning collecting men when he rises, or from the phrase ἀεὶ εἱλεῖν, aeí heileín, "ever turning" because he always turns the earth in his course.

Doric Greek retained Proto-Greek long *ā as α, while Attic changed it in most cases, including in this word, to η. Cratylus and the etymologies Plato gives are contradicted by modern scholarship. From helios comes the modern English prefix helio-, meaning "pertaining to the Sun", used in compounds word such as heliocentrism, aphelion, heliotropium, heliophobia (fear of the sun) and heliolatry ("sun-worship").

Helios most likely is Proto-Indo-European in origin. Walter Burkert wrote that "... Helios, the sun god, and Eos-Aurora, the goddess of the dawn, are of impeccable Indo-European lineage both in etymology and in their status as gods" and might have played a role in Proto-Indo-European poetry. The imagery surrounding a chariot-driving solar deity is likely Indo-European in origin. Greek solar imagery begins with the gods Helios and Eos, who are brother and sister, and who become in the day-and-night-cycle the day (hemera) and the evening (hespera), as Eos accompanies Helios in his journey across the skies. At night, he pastures his steeds and travels east in a golden boat. In them evident is the Indo-European grouping of a sun god and his sister, as well as an association with horses.

Helen of Troy's name is thought to share the same etymology as Helios, and she may express an early alternate personification of the sun among Hellenic peoples. Helen might have originally been considered to be a daughter of the Sun, as she hatched from an egg and was given tree worship, features associated with the Proto-Indo-European Sun Maiden; in surviving Greek tradition however Helen is never said to be Helios's daughter, instead being the daughter of Zeus.