He was reportedly son of Erechtheus, King of Athens, and Praxithea. His maternal grandparents were Phrasimus and Diogenia, the daughter of the river god Cephissus. He married Megamede, daughter of Arneus. They supposedly had fifty daughters together, although Thespius may have fathered some of the daughters from unnamed mistresses with Megamede being their stepmother. The daughters are often referred to as the Thespiades, also being the subject of an 1853 painting by Gustave Moreau.
All his daughters came of marrying age but Thespius seems to have sought no husband for them; he instead desired grandchildren from the hero Heracles. When Heracles was assigned to kill a lion (not to be confused with the Nemean Lion), Thespius offered his fifty daughters as a prize. The hunt for the lion lasted fifty days, and during each night of the hunt Heracles slept with each of the fifty daughters, who in turn each gave birth to one son.
Alternate sources claim that Heracles slept with the daughters in a single night. In this version, only forty-nine slept with the hero, with the fiftieth being destined to serve as a virgin priestess of a temple to Heracles, as a punishment for her refusal to sleep with him. In another version there were fifty-one grandsons of Thespius, of which forty colonized the island of Sardinia.
Daughters and grandchildren
- Aeschreis. Mother of Leucones.
- Aglaia or Aeglaea. Mother of Antiades.
- Anthea. Unknown child.
- Anthippe. Mother of Hippodromus.
- Antiope. Mother of Alopius.
- Argele. Mother of Cleolaus.
- Asopis. Mother of Mentor.
- Calametis. Mother of Astybies.
- Certhe. Mother of Iobes.
- Chryseis. Mother of Onesippus.
- Clytippe. Mother of Eurycapys.
- Elachia. Mother of Buleus.
- Eone. Mother of Amestrius.
- Epilais. Mother of Astyanax.
- Erato. Mother of Dynastes.
- Euboea. Mother of Olympus and grandmother of Marsyas.
- Eubote. Mother of Eurypylus.
- Eurybia. Mother of Polylaus.
- Euryce (Euryte?). Mother of Teleutagoras.
- Eurypyle. Mother of Archedicus.
- Eurytele. Mother of Leucippus.
- Exole. Mother of Erythras.
- Heliconis. Mother of Phalias.
- Hesychia. Mother of Oestrobles.
- Hippo. Mother of Capylus.
- Hippocrate. Mother of Hippozygus.
- Iphis. Mother of Celeustanor.
- Laothoe. Mother of Antiphus.
- Lyse. Mother of Eumedes.
- Lysidice. Mother of Teles.
- Lysippe. Mother of Erasippus.
- Marse. Mother of Bucolus.
- Meline. Mother of Laomedon.
- Menippis. Mother of Entelides.
- Nice. Mother of Nicodromus.
- Nicippe. Mother of Antimachus.
- Olympusa. Mother of Halocrates.
- Oria. Mother of Laomenes.
- Panope. Mother of Threpsippas.
- Patro. Mother of Archemachus.
- Phyleis. Mother of Tigasis.
- Praxithea. Mother of Nephus.
- Procris. Mother of Antileon and Hippeus.
- Pyrippe. Mother of Patroclus.
- Stratonice. Mother of Atromus.
- Terpsicrate. Mother of Euryopes.
- Tiphyse. Mother of Lyncaeus.
- Toxicrate. Mother of Lycurgus.
- Xanthis. Mother of Homolippus.
- An unnamed daughter. Mother of Creon.
- Pausanias, Description of Greece, 9. 26. 6; scholia on Homer, Iliad, 2. 498 call him son of Teuthras or Cepheus
- Pseudo-Apollodorus, Bibliotheca 3. 15. 1
- So in Diodorus Siculus, Library of History, 4. 29. 2
- Bibliotheca 2. 4. 10
- Pausanias, Description of Greece, 9. 27. 7
- Diodorus Siculus, Library of History, 4. 29. 1, 4–6
- Pseudo-Apollodorus, Bibliotheca 2. 7. 8