In Greek mythology, Thoas was a son of the god Dionysus and Ariadne, the daughter of Cretan king Minos. Some, however, consider him to be Theseus’s son, together with his brother Oenopion. Rhadamanthus, Ariadne's uncle, bequeathed Thoas the island of Lemnos, over which he reigned until his daughter Hypsipyle, unable to kill her own father to avenge the offenses against the Lemnian women, tied him secretly in an oarless boat and sent him adrift into the Aegean Sea. He was carried to the island of Oinoie, where he consorted with the nymph of the island, also called Oinoie, and had by her a son Sicinus, who later had the island renamed after himself. Thoas eventually arrived at Tauris, in the Crimea, where he was made king and where Artemis installed Agamemnon's daughter Iphigeneia as her temple's priestess.
In the play Iphigenia in Tauris, Iphigenia is ordered to execute her brother, Orestes, after he tries to steal a statue of Artemis. When the siblings discover each other's identity, they discuss ways to escape. Orestes wants to kill King Thoas, but Iphigenia suggests that they trick him. Iphigenia meets Thoas and claims that the statue has been tainted with the sin of matricide. She says that the statue has turned and closed its eyes. Thoas believes her and allows her to wash Orestes and his traveling companion Pylades. He even gives her freedom to do it in privacy, giving the order that everybody should stay indoors. Iphigenia, Orestes, Pylades, and the Greek slaves all escape with the help of Athena. A messenger relays this to Thoas and he immediately sends his men to attack. Athena intervenes and convinces him to let them go.