In Greek mythology, Atreus (Ancient Greek: Ἀτρεύς, [a.trěu̯s] lit. 'fearless') was a king of Mycenae in the Peloponnese, the son of Pelops and Hippodamia, and the father of Agamemnon and Menelaus. His descendants became known collectively as the Atreidae (Ancient Greek: Ἀτρείδαι Atreidai).

Atreus and his brother Thyestes were exiled by their father for murdering their half-brother Chrysippus in their desire for the throne of Olympia. They took refuge in Mycenae, where they ascended to the throne in the absence of King Eurystheus, who was fighting the Heracleidae. Eurystheus had meant for their stewardship to be temporary, but it became permanent after his death in battle, which ended the rule of the Perseid dynasty in Mycenae.

According to most ancient sources, Atreus was the father of Pleisthenes, but in some lyric poets (Ibycus, Bacchylides) Pleisthenides (son of Pleisthenes) is used as an alternative name for Atreus himself.

'Atreides' (Ἀτρείδης) is a patronymic form of Atreus which refers to one of his sons—Agamemnon or Menelaus. The plural forms, deriving from the Latin Atreidae, itself from Ancient Greek 'Atreidai' (Ἀτρεῖδαι), refer to both sons collectively. 'Atreides' is commonly used to translate both the singular and plural form to English. The term can also be used for the more distant descendants of Atreus, also known as the 'House of Atreus' or the 'Atreid dynasty'. It has been suggested that the name Attarsiya, belonging to an Ahhiyawan king of 1400 BC, may be the Hittite rendition of the Greek name.

The 'House of Atreus' begins with Tantalus. Tantalus, the son of Zeus and the maiden Pluto, enjoyed cordial relations with the gods until he decided to slay his son Pelops and feed him to the gods as a test of their omniscience. Most of the gods, as they sat down to dinner with Tantalus, immediately understood what had happened, and, because they knew the nature of the meat they were served, were appalled and did not partake. But Demeter, who was distracted due to the abduction by Hades of her daughter Persephone, obliviously ate Pelops's shoulder. The gods threw Tantalus into the underworld to spend eternity standing in a pool of water beneath a fruit tree with low branches. Whenever he reaches for the fruit, the branches raise his intended meal from his grasp. Whenever he bends down to get a drink, the water recedes before he can drink. Thus is derived the word "tantalizing" in English. The gods brought Pelops back to life, replacing the bone in his shoulder with a bit of ivory with the help of Hephaestus, thus marking the family forever afterwards.

Pelops married Hippodamia after winning a chariot race against her father, King Oenomaus, by arranging for the sabotage of his would-be-father-in-law's chariot which resulted in his death, at which point the story diverges into multiple versions. The sabotage was arranged by Myrtilus, a servant of the king who was killed by Pelops for one of three reasons: 1) because he had been promised the right to take Hippodamia's virginity, which Pelops retracted; 2) because he attempted to rape her, or; 3) because Pelops did not wish to share the credit for the victory. As Myrtilus died, he cursed Pelops and his line, further adding to the house's curse.

Pelops and Hippodamia had many sons; two of them were Atreus and Thyestes. Depending on myth versions, they murdered Chrysippus, who was their half-brother. Because of the murder, Hippodamia, Atreus, and Thyestes were banished to Mycenae, where Hippodamia is said to have hanged herself.

Atreus vowed to sacrifice his best lamb to Artemis. Upon searching his flock, however, Atreus discovered a golden lamb which he gave to his wife, Aerope, to hide from the goddess. She gave it to Thyestes, her lover and Atreus' brother, who then persuaded Atreus to agree that whoever had the lamb should be king. Thyestes produced the lamb and claimed the throne.