Hyacinth received various parentage assignments reflecting local connections. Sources identify him as the son of Clio and Pierus, or of King Oebalus of Sparta, or of King Amyclus of Sparta.

In one tradition he is the youngest and most beautiful son of Amyclas and Diomede (daughter of Lapithes). In this account he had numerous siblings, including Cynortus, Argalus, Polyboea, Laodamia (or Leanira), Harpalus, Hegesandra, and, in some versions, Daphne.

In Greek mythology, Hyacinth was a Spartan prince of remarkable beauty and Apollo's lover. He was also admired by Zephyrus (god of the west wind), Boreas (god of the north wind), and the mortal Thamyris. Hyacinth chose Apollo, accompanying the god across his sacred lands in a swan-drawn chariot. Apollo's devotion was so intense that he abandoned his Delphi sanctuary to spend time with Hyacinth by the river Eurotas, teaching him archery, lyre-playing, prophecy, and gymnasium exercises.

During a friendly discus competition, Apollo accidentally killed Hyacinth. According to Ovid's account, both lovers stripped and oiled themselves for the contest; Apollo threw first with tremendous force, splitting the clouds, and as Hyacinth ran to retrieve the discus it bounced back and struck his head fatally. An alternative version credits Zephyrus, who — jealous that Hyacinth preferred Apollo — deliberately blew the discus off course.

Apollo's face grew pale as he held his dying lover. In Bion of Smyrna's poem, Apollo tried to heal him with herbs and ambrosia, but the wound inflicted by the Fates proved incurable. Apollo wept and wished to become mortal so as to accompany Hyacinth in death; since immortality prevented this, he promised perpetual remembrance through his songs and lyre music, and transformed Hyacinth's spilled blood into a flower bearing the lamentation inscription "AI AI" ("alas") on its petals. Scholars note that this ancient flower differs from the modern hyacinth plant.

The Bibliotheca credited Thamyris as the first man to experience romantic love for another man, on the basis of his feelings toward Hyacinth.

Hyacinth was eventually resurrected by Apollo and achieved immortality. Pausanias recorded that Apollo's throne in Sparta depicted a bearded Hyacinth being taken to heaven by Aphrodite, Athena, and Artemis, accompanied by Polyboea.

Hyacinth served as the tutelary deity of the Hyacinthia, one of Sparta's principal festivals, celebrated during the Spartan month of Hyacinthia in early summer. This three-day festival combined mourning for his death with celebration of his rebirth, though scholarly debate continues over the precise distribution of these honors.

The first day involved mourning: citizens ate minimally and refrained from singing, unlike at other Apollo festivals. The second day featured choirs of boys and young men performing national songs and dancing, while girls were either carried in decorated wicker carts or paraded in horse-drawn chariots that raced; citizens entertained friends and servants alike. Each year Laconian women wove a chiton for Apollo and presented it — a tradition paralleling the Athenian peplos offered to Athena at the Panathenaic Games. The third day, a "merry midnight festival," likely involved mysteries, with few historical details surviving.

The festival was so important that the Amyclaeans returned home from military campaigns during the Hyacinthia season; the Lacedaemonians even negotiated a forty-day truce with Eira to celebrate it, and after the 421 BC treaty with Sparta the Athenians promised annual attendance as a gesture of goodwill.

Hyacinth shared a shrine at Amyclae with Apollo. Scholars widely agree he was a pre-Hellenic nature deity, with cult aspects suggesting a chthonic vegetation god whose worship merged with Apollo's. Hyacinth personifies sprouting spring vegetation destroyed by summer heat, and his apotheosis signifies the natural cycle of decay and renewal.

At Tarentum, Polybius attributed a Hyacinth tomb to "Apollo Hyacinthus" (not Hyacinth alone), leading some scholars to theorize that the two personalities are unified and that the hero merely emanates from the god. The Laconian appellation "τετράχειρ" (reportedly used by Hesychius) supposedly describes an Apollo-Hyacinth composite figure. Yet essential differences contradict this theory: Hyacinth is a chthonian vegetation god whose devotees experience sorrow, whereas Apollo's worship is joyous and triumphant and his death receives no ritual celebration; the Amyclean Apollo specifically governs war and song. Pausanias, moreover, describes the Amyclae monument as a crude Apollo figure standing on an altar-shaped base that forms Hyacinth's tomb, with offerings placed for the hero before gifts to the god.

Apollo gave Hyacinth a swan chariot, and Hyacinth appears in ancient art riding it, either meeting Apollo or escaping Zephyrus's advances. Swans were believed to be the birds of Hyperborea — a mystical land of eternal spring and immortality to which Apollo himself traveled yearly on a swan-drawn chariot each winter. This swan association links Hyacinth closely to the Hyperborean Apollo and to spring renewal, and scholars suggest he spent the winter months in the underworld or Hyperborea, returning to earth each spring when the hyacinth flowers bloom. Classical interpretations read his myth as nature's death-and-rebirth metaphor, and the Hyacinthia included initiatory rites marking youths' transition into adulthood.

The hyacinth flower said to have arisen from Hyacinth's blood reportedly displayed a deep blue or purple coloring with "AI" markings symbolizing sorrow. This flower has been identified with larkspur, iris, or possibly Gladiolus italicus rather than the modern hyacinth. Other divinely beloved vegetation figures who died young and underwent botanical transformation include Narcissus, Cyparissus, Mecon, and Adonis.

Ancient Greeks associated Apollo with a deep blue or violet precious gem called hyacinth, named for its resemblance to the flower's color. The gem was sacred to Apollo because of these mythological connections, and visitors to his shrine, his priests, and the high priestess Pythia were required to wear it.

"Hyacinthine hair" describes Hyacinth's curly locks, resembling the curled form of the hyacinth flower's petals, and is frequently used poetically. The term could also describe hair color, either dark or deep violet. In Homer's Odyssey, Athena grants Odysseus hyacinthine hair to enhance his beauty, and Edgar Allan Poe employs the term in "To Helen" to beautify Helen's hair.