Aegisthus (; ; also transliterated as Aigisthos) is a figure in Greek mythology. He was the son of Thyestes and his daughter, Pelopia. The product of an incestuous union motivated by his father's rivalry with the house of Atreus for the throne of Mycenae, Aegisthus murdered Atreus to restore his father to power. Later, he lost the throne to Atreus's son Agamemnon.