In Greek mythology, Themisto (; Ancient Greek), daughter of Hypseus, was the third and last wife of Athamas. According to some sources, she had four children by him: Leucon, Erythrius, Schoeneus, and Ptous. In other sources there were but two: Sphincius and Orchomenus, or else Schoeneus and Leucon. Some say that the father of Leucon was Poseidon (see also Leuconoe). Her name is derived from the Ancient Greek word: "" which means "belonging to the law", or "belonging to the customs".
Themisto intended to kill her husband's children by his previous wife, but accidentally slew her own sons. This was the subject of a non-surviving tragedy by Euripides, retold by Hyginus as follows. Athamas married Themisto as he believed his second wife, Ino, was dead, but Ino turned out to be alive and to have been on Mount Parnassus with the Maenads. Athamas had her brought home but kept her return a secret; Themisto did find out she was back, and resolved to kill Ino's children as an act of revenge. However, she had never seen Ino in person and took her for a servant as they met, and ordered the "servant" to dress all her own children in white clothing, and Ino's in black. Themisto then proceeded to kill all the black-clothed children. What Themisto did not realise was that Ino had switched the children's clothing, and so she in fact killed her own children. Upon discovering that, she killed herself. According to Pseudo-Apollodorus, however, Themisto married Athamas after the death of Ino, and the whole story with the murder of the children did not take place.