He was the son of Oeneus and either Periboea or Gorge. When he was young, he was banished from his hometown of Calydon by his uncle, after murdering one of his family members. He found refuge at the court of King Adrastus of Argos, where he married Deipyle, Adrastus' daughter. Tydeus and Deipyle had a son, whom they named Diomedes.
At the court of Adrastus, Tydeus met Polynices, the son of Oedipus, former king of Thebes. Polynices and his brother Eteocles had decided to rule Thebes on an alternating basis, but when it was time for Eteocles to step down, he instead exiled his brother. One night, Tydeus and Polynices argued and started fighting; Adrastus tried to stop the fight. That's when he realised that the prophecy he had been given about giving his daughters to a boar and a lion, was talking about his guests. So, he arranged a wedding between the two guests and his daughters.
Adrastus also promised Polynices to give him back his throne. So, with the help of Tydeus as well, seven armies were raised in order to take back Thebes for Polynices. These were the Seven Against Thebes. Before the armies marched against Thebes, Tydeus was sent as a diplomatic envoy to demand the reinstating of Polynices to the throne. After the refusal from Eteocles, Tydeus returned to his army, and the war began. After a fierce battle, the outcome was victorious for the city of Thebes, while all seven attacking leaders were killed.