Aethra was the mother of Theseus in Greek mythology. She was the daughter of King Pittheus of Troezen (and thus granddaughter of Pelops). She lay with both the visiting Aegeus (King of Athens) and Poseidon in one night and later gave birth to Theseus.
She later look after and attended Helen, who was kidnapped by Theseus. She was kidnapped, along with Helen, by Helen's brothers Castor and Pollux in revenge. She remained in Sparta and eventually accompanied Helen to Troy, where she witnessed the fall of the city to Greek troops and where she was found by her grandson Demophon.