The Hyrcanian plain (Greek: Ὑρκάνιον πεδίον) was a plain near Sardis, the provincial capital of the Achaemenid Persian satrapy (province) of Lydia. Located in the Hermos valley, in what is present-day western Turkey, according to the ancient geographer and historian Strabo, the plain derived its name from a colony of Hyrcanians who were settled there by the Achaemenids. According to the modern historian and archaeologist Nicholas Victor Sekunda, the plain was either "confined to the valley of the Pidasos, or it extended in a broad band along the Phrygios from Thyateira to Magnesia-on-Sipylos to the west, and possibly even as far as Sardis to the southeast".
The centre of worship for the Iranian settlers in the Hyrcanian plain was likely the shrine at Hierakome, located almost at the plain's centre. The shrine was probably founded by the first Achaemenid King Cyrus the Great (r. 559–530 BC).