In Greek mythology, Hypnos (/ˈhɪpnɒs/; Ancient Greek: Ὕπνος, 'sleep'), also spelled Hypnus, is the personification of sleep. The Roman equivalent is Somnus. His name is the origin of the word hypnosis. Pausanias wrote that Hypnos was the dearest friend of the Muses.

According to the Dutch linguist Robert S. P. Beekes, the god's name derives from the Proto-Indo-European root *sup-no- 'sleep'.

Hypnos is usually the fatherless son of Nyx ("The Night"), although sometimes Nyx's consort Erebus ("The Darkness") is named as his father. His twin brother is Thanatos ("Death"). Both siblings live in the underworld (Hades). According to rumors, Hypnos lived in a big cave, which the river Lethe ("Forgetfulness") comes from and where night and day meet. They call this area the Land of Dreams. His bed is made of ebony, and on the entrance of the cave grow several poppies and other soporific plants. No light and no sound would ever enter his grotto. According to Homer, he lives on the island Lemnos, which later on has been claimed to be his very own dream island. He is said to be a calm and gentle god, as he helps humans in need and, due to their sleep, owns half of their lives.

Hypnos lived next to his twin brother, Thanatos (Θάνατος, 'death'), in the underworld, where the rays of the sun never reached them.

In Hesiod's Theogony, Hypnos is one of the offspring of Nyx (Νύξ, 'Night'), the goddess of Night, without a father. In genealogies from works by Roman authors, he is the son of Erebus (Darkness) and Nox (Night, the Roman name for Nyx). In the Iliad, Nyx is a dreadful and powerful goddess, and even Zeus fears to enter her realm.

His wife, Pasithea, is one of the youngest of the Charites and is promised to him by Hera, who is the goddess of marriage and birth.

Hypnos was able to trick Zeus and help the Danaans win the Trojan War. During the war, Hera loathed her brother and husband, Zeus, so she devised a plot to trick him. She decided that to trick him she needed to make him so enamored with her that he would fall for the trick. So she washed herself with ambrosia and anointed herself with oil, made especially for her to make herself impossible for Zeus to resist. She wove flowers through her hair, put on three brilliant pendants for earrings, and donned a wondrous robe. She then called for Aphrodite, the goddess of love, and asked her for a charm that would ensure that her trick would not fail. To procure the charm, however, she lied to Aphrodite because they sided on opposite sides of the war. She told Aphrodite that she wanted the charm to help herself and Zeus stop fighting. Aphrodite willingly agreed. Hera was almost ready to trick Zeus, but she needed the help of Hypnos, who had tricked Zeus once before.

Hera called on Hypnos and asked him to help her by putting Zeus to sleep. Hypnos was reluctant because the last time he had put the god to sleep, he was furious when he awoke. It was Hera who had asked him to trick Zeus the first time as well. She was furious that Heracles, Zeus' son, sacked the city of the Trojans. So she had Hypnos put Zeus to sleep, and set blasts of angry winds upon the sea while Heracles was still sailing home. When Zeus awoke he was furious and went on a rampage looking for Hypnos. Hypnos managed to avoid Zeus by hiding with his mother, Nyx. This made Hypnos reluctant to accept Hera's proposal and help her trick Zeus again. Hera first offered him a beautiful golden seat that can never fall apart and a footstool to go with it. He refused this first offer, remembering the last time he tricked Zeus. Hera finally got him to agree by promising that he would be married to Pasithea, one of the youngest Graces, whom he had always wanted to marry. Hypnos made her swear by the river Styx and call on the gods of the underworld to be witnesses so that he would be ensured that he would marry Pasithea.