In Greek mythology, Danaus (, ; Ancient Greek: Δαναός, romanized: Danaós) was the king of Libya. His myth is a foundation legend of Argos, one of the foremost Mycenaean cities of the Peloponnesus. In Homer's Iliad, "Danaans" ("tribe of Danaus") and "Argives" commonly designate the Greek forces opposed to the Trojans.

Danaus, was the son of King Belus of Egypt and the naiad Achiroe, daughter of the river god Nilus, or of Sida, eponym of Sidon. He was the twin brother of Aegyptus, king of Egypt while Euripides adds two others, Cepheus, king of Ethiopia and Phineus, betrothed of Andromeda.

Danaus had fifty daughters, the Danaides, twelve of whom were born to the naiad Polyxo; six to Pieria; two to Elephantis; four to Queen Europa; ten to the hamadryad nymphs Atlanteia and Phoebe; seven to an Aethiopian woman; three to Memphis; two to Herse and lastly four to Crino. According to Hippostratus, Danaus had all these progenies begotten by Europa, the daughter of Nilus. In some accounts, Danaus married Melia while Aegyptus consorted with Isaie, these two women were daughters of their uncle Agenor, King of Tyre, and their possible sister, Damno who was described as the daughter of Belus.

After Aegyptus commanded that his fifty sons should marry the Danaides, Danaus elected to flee instead. To that purpose, he built a ship on the advice of Athena, the first ship that ever was. In it, he fled to Argos, to which he was connected by his descent from Io, a priestess of Hera at Argos, who was wooed by Zeus and turned into a heifer and pursued by Hera until she found asylum in Egypt. Argos at the time was ruled by King Pelasgus, the eponym of all autochthonous [indigenous] inhabitants who had lived in Greece since the beginning, also called Gelanor ("he who laughs"). The Danaides asked Pelasgus for protection when they arrived, the event portrayed in The Suppliants by Aeschylus. Protection was granted after a vote by the Argives. When Pausanias visited Argos in the 2nd century CE, he related the succession of Danaus to the throne, judged by the Argives, who "from the earliest times ... have loved freedom and self-government, and they limited to the utmost the authority of their kings":

"On coming to Argos he claimed the kingdom against Gelanor, the son of Sthenelus. Many plausible arguments were brought forward by both parties, and those of Sthenelas were considered as fair as those of his opponent; so the people, who were sitting in judgment, put off, they say, the decision to the following day. At dawn a wolf fell upon a herd of oxen that was pasturing before the wall, and attacked and fought with the bull that was the leader of the herd. It occurred to the Argives that Gelanor was like the bull and Danaus like the wolf, for as the wolf will not live with men, so Danaus up to that time had not lived with them. It was because the wolf overcame the bull that Danaus won the kingdom. Accordingly, believing that Apollo had brought the wolf on the herd, he founded a sanctuary of Apollo Lycius." The sanctuary of Apollo Lykeios ("wolf-Apollo", but also Apollo of the twilight) was still the most prominent feature of Argos in Pausanias's time: in the sanctuary, the tourist might see the throne of Danaus himself, an eternal flame, called the fire of Phoroneus.

When Aegyptus and his fifty sons arrived to take the Danaides, Danaus gave them in order to spare the Argives the pain of a battle. However, he instructed his daughters to kill their husbands on their wedding night. Forty-nine followed through and subsequently buried the heads of their bridegrooms in Lerna; but one, Hypermnestra, refused because her husband, Lynceus, honored her wish to remain a virgin. Danaus was angry with his disobedient daughter and threw her to the Argive courts. Aphrodite intervened and saved her. Lynceus and Hypermnestra then began a dynasty of Argive kings (the Danaid Dynasty).