In Greek mythology, Menelaus (/ˌmɛnəˈleɪ.əs/; Ancient Greek: Μενέλαος) was a Greek king of Mycenaean (pre-Dorian) Sparta. According to the Iliad, the Trojan war began as a result of Menelaus's wife, Helen, fleeing to Troy with the Trojan prince Paris. Menelaus was a central figure in the Trojan War, leading the Spartan contingent of the Greek army, under his elder brother Agamemnon, king of Mycenae. Prominent in both the Iliad and Odyssey, Menelaus was also popular in Greek vase painting and Greek tragedy, the latter more as a hero of the Trojan War than as a member of the doomed House of Atreus.

In the account of Dares the Phrygian, Menelaus was described as "of moderate stature, auburn-haired, and handsome. He had a pleasing personality."

Menelaus was a descendant of Pelops son of Tantalus. He was the younger brother of Agamemnon, and the husband of Helen of Troy. According to the usual version of the story, followed by the Iliad and Odyssey of Homer, Agamemnon and Menelaus were the sons of Atreus, king of Mycenae, and Aerope, daughter of the Cretan king Catreus. However, according to another tradition, Agamemnon and Menelaus were the sons of Atreus's son Pleisthenes, with their mother being Aerope, Cleolla, or Eriphyle. According to this tradition Pleisthenes died young, with Agamemnon and Menelaus being raised by Atreus. Agamemnon and Menelaus had a sister Anaxibia (or Astyoche) who married Strophius, the son of Crisus.

According to the Odyssey, Menelaus had only one child by Helen, a daughter named Hermione; and an illegitimate son, Megapenthes, by a slave. Other sources mention other sons of Menelaus by either Helen, or slaves. A scholiast on Sophocles's Electra quotes Hesiod as saying that after Hermione, Helen also bore Menelaus a son Nicostratus, while according to a Cypria fragment, Menelaus and Helen had a son Pleisthenes. The mythographer Apollodorus, tells us that Megapenthes's mother was a slave "Pieris, an Aetolian, or, according to Acusilaus, ... Tereis", and that Menelaus had another illegitimate son Xenodamas by another slave girl, Cnossia, while according to the geographer Pausanias, Megapenthes and Nicostratus were sons of Menelaus by a slave. The scholiast on Iliad 3.175 mentions Nicostratus and Aethiolas as two sons of Helen (by Menelaus?) worshipped by the Lacedaemonians and another son of Helen by Menelaus, Maraphius, from whom descended the Persian Maraphions.

Although early authors, such as Aeschylus, refer in passing to Menelaus's early life, detailed sources are quite late, post-dating 5th-century BC Greek tragedy. According to these sources, Menelaus's father, Atreus, had been feuding with his brother Thyestes over the throne of Mycenae. After a back-and-forth struggle that featured adultery, incest, and cannibalism, Thyestes gained the throne after his son Aegisthus murdered Atreus. As a result, Atreus's sons, Menelaus and Agamemnon, went into exile. They first stayed with King Polypheides of Sicyon, and later with King Oeneus of Calydon. But when they thought the time was ripe to dethrone Mycenae's hostile ruler, they returned. Assisted by King Tyndareus of Sparta, they drove Thyestes away, and Agamemnon took the throne for himself.

When it was time for Tyndareus's stepdaughter Helen to marry, many kings and princes came to seek her hand. Among the contenders were Odysseus, Menestheus, Ajax the Great, Patroclus, and Idomeneus. Most offered opulent gifts. Tyndareus would accept none of the gifts, nor would he send any of the suitors away for fear of offending them and giving grounds for a quarrel. Odysseus promised to solve the problem in a satisfactory manner if Tyndareus would support him in his courting of Tyndareus's niece Penelope, the daughter of Icarius. Tyndareus readily agreed, and Odysseus proposed that, before the decision was made, all the suitors should swear a most solemn oath to defend the chosen husband in any quarrel. Then it was decreed that straws were to be drawn for Helen's hand. The suitor who won was Menelaus (Tyndareus, not to displease the mighty Agamemnon offered him another of his daughters, Clytaemnestra). The rest of the suitors swore their oaths, and Helen and Menelaus were married, Menelaus becoming a ruler of Sparta with Helen after Tyndareus and Leda abdicated the thrones.

Their supposed palace (ἀνάκτορον) has been discovered (the excavations started in 1926 and continued until 1995) in Pellana, Laconia, to the north-west of modern (and classical) Sparta. Other archaeologists consider that Pellana is too far away from other Mycenaean centres to have been the "capital of Menelaus".

According to tradition Menelaus founded the port-city Menelai Portus on the coast of Marmarica in Northern Africa.