The Treasury of Athena on the Acropolis in Athens was originally located in a room known as the Opisthodomos in the Old Temple of Athena Polias (the Archaios Neos). After 406/5 B.C.E., it was moved to the western room of the Temple of Athena Parthenos. The evidence for this interpretation comes from the inventories of the Temple of Athena Parthenos. These inventories, dated from 434/3 to ca. 408/7 B.C.E., record offerings housed in three places in the new temple: the Proneos (the eastern porch), the Hekatompedon (the cella), and the Parthenon. This “Parthenon” must refer to the chamber behind the cella, which is entered from the west and which housed a collection of miscellaneous sacred objects. In 406/5, there was a fire in the Old Temple, and in 403/2, inventories recording dedications “from the Opisthodomos” and “from the Parthenon” first appear. They show that both the Opisthodomos and the room called the Parthenon were emptied, most likely as a consequence of the fire. This article argues that after the western room in the Temple of Athena Parthenos, which had been called the Parthenon in the earlier inventories, was cleared, it was used as the treasury and renamed the Opisthodomos.