Proetus was a king of the cities of Argos and Tiryns in Greek mythology, son of Abas and Aglaea. His twin brother was Acrisius, with whom they quarreled all the time. This rivalry escalated over the years, and Proetus even seduced Acrisius' daughter, Danae. He had the throne of Argos for seventeen years, but Acrisius managed to win the throne and exile his brother. Proetus went to Lycia, where he married Aglaea, the daughter of King Jobates. The king later helped Proetus by starting a war against Acrisius, which led to Acrisius and Proetus sharing the kingdom.
Proetus and Aglaea had three daughters, Lysippe, Iphinoe and Iphianassa; collectively known as the Proitides; and a son, Megapenthes. The Proitides, at some point, were inflicted with madness; according to a source, this was because they scorned the worship of god Dionysus, while a different source mentions they considered themselves more beautiful than Hera and were punished for that. Proetus was told by Melampus that he could cure them if he received one third of the kingdom, but Proetus refused. His daughters' madness increased and even spread to other women in the area. Proetus eventually agreed to Melampus' terms, who now asked for another third of the kingdom for his brother Bias. The two brothers, along with a small army of men, tracked down the women, and eventually cured two of Proetus' daughters, while Iphinoe died while being chased.
Later, the hero Bellerophon arrived to Proetus' court in order to be cleansed of a murder he had committed. Aglaea fell in love with him, but he rejected her proposals. Angry, she accused Bellerophon of being indecent to her, and Proetus sent Bellerophon to King Jobates, along with a letter which asked the king to kill Bellerophon. Jobates gave Bellerophon a series of impossible tasks which the hero successfully completed.
One source mentions that Proetus eventually died when he was turned to stone by looking at Medusa's head when Perseus presented it at the court of Polydectes. In the end, Megapenthes, Proetus' son, exchanged the kingdoms with Perseus.