Trophonius (the Latinate spelling) or Trophonios (the transliterated Greek spelling of Τροφώνιος) was a Greek hero or daimon or god—it was never certain which one—with a rich mythological tradition and an oracular cult at Lebadaea in Boeotia.

The name is etymologically derived from τρέφω trepho, "to nourish". Strabo and several inscriptions refer to him as Zeus Trephonios. Several other chthonic Zeuses with similar titles are known from the Greek world, including Zeus Μειλίχιος Meilikhios ("honeyed" or "kindly" Zeus), and Zeus Χθόνιος Chthonios ("Zeus beneath-the-earth").

Similar constructions are also found in the Roman world: for example, a shrine at Lavinium in Lazio was dedicated to Aeneas under the title Iuppiter Indiges (Jupiter in-the-earth).

In Greek mythology, Trophonius was a son of Erginus. According to the Homeric Hymn to Apollo, he built Apollo's temple at the oracle at Delphi with his brother, Agamedes. Once finished, the oracle told the brothers to do whatsoever they wished for six days and, on the seventh, their greatest wish would be granted. They did and were found dead on the seventh day. The saying "those whom the gods love die young" comes from this story.