In Greek mythology, Aethra was a daughter of King Pittheus of Troezena and, with Aegeas, or in some versions, Poseidon, mother of Theseus.

Aegeas (King of Athens) went to Troezena (a city located SW of Athens) and met with Aethra, daughter of Troezena's king, Pittheus. They had sexual intercourse and then, in some versions, Aethra waded out to the sea to Sphairia and became pregnant through Poseidon. When she got pregnant, Aegeas decided to go back to Athens. Before leaving, he covered his sandals, shield and sword under a huge rock and told her that when their son would grow up, he should move the rock and bring his weapons back. Aethra did as she was told and Theseus grew up to be a great hero, killing the Minotaur, among having other adventures.

Later, Theseus kidnapped Helen and gave her to Aethra to keep. Helen's brothers, the Dioscuri, took Helen back and kidnapped Aethra in revenge.