In Greek mythology, Thespius (/ˈθɛspiəs/; Ancient Greek: Θέσπιος Théspios) or Thestius (/ˈθɛstʃəs, ˈθɛstiəs/; Ancient Greek: Θέστιος)^[1][2] was a legendary founder and king of Thespiae, Boeotia. His life account is considered part of Greek mythology.

Thespius was reportedly son of Erechtheus, King of Athens,^[3] and possibly Praxithea, daughter of Phrasimus and Diogeneia.^[4] He was probably the brother of Protogeneia, Pandora, Procris, Creusa, Oreithyia, Chthonia,^[5] Merope,^[6] Cecrops, Pandorus, Metion,^[4] Orneus,^[7] Eupalamus^[8] and Sicyon.^[9] Other sources called him a descendant of Erechtheus^[10] while some said that he was the son of Teuthras, son of Pandion^[11] or Cepheus.^[12]

Thespius' maternal grandparents were Phrasimus and Diogenia, the daughter of the river god Cephissus.^[13] 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.^[3] The daughters are often referred to as the Thespiades,^[14] 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.^[15]

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.^[1] In another version, there were fifty-one grandsons of Thespius, forty of which colonized the island of Sardinia.^[16]

The Bibliotheca of Pseudo-Apollodorus^[17] lists the following daughters and grandchildren. The grandchildren were all Heracleidae in the wider sense of the term.

According to Hellanicus, a certain Sthephanephoros ("crown bearer") was called one of the sons of Heracles who were born from the daughters of Thestios.^[18]