In Greek mythology, Iphimedeia (Ἰφιμέδεια) was the daughter of Triopas of Thessaly (a son of Poseidon and Canace). Her brothers were Erysichthon and Phorbas. She was the wife of Aloeus, who was also her uncle. She is attested in Homer's Odyssey in the catalogue of women as being a mortal.
With Aloeus, Iphimedeia had a daughter Pancratis (or Pancrato), renowned for her beauty. When she and her daughter were participating in the celebration of the orgies of Dionysus, they were carried off by the companions of the Thracian Butes and brought to the island of Strongyle (later Naxos), where Pancratis was given in marriage to the new king Agassamenus and Iphimedia to a friend and lieutenant of his. Two other leaders, Sicelus and Hecetorus, had fought over Pancratis and killed each other (or else they were Scellis and Agassamenus himself). Soon after, Otus and Ephialtes, sent by Aloeus, defeated the Thracians and rescued their mother and sister; but Pancratis died not much later.
Iphimedeia also fell in love with Poseidon and would often come to the sea shore and pour the sea water in her lap, until the god came and answered her feelings (cf. the story of Tyro). With Poseidon she was the mother of Otus and Ephialtes (who were called the Aloadae after their stepfather), as well as Cercyon and the bandit Sciron. One account calls Aloeus natural father of the Aloadae.
According to Pietro Scarpi, Iphimedeia should be placed in the chthonic realm as a double of Hekate.
Iphimedeia was one of the heroines whose spirits Odysseus encountered at the entrance of the Underworld.