The sanctuary of Apollo Delphinios is a rectangular temenos enclosure bordered by two-aisled stoas at the north, east and south. The sanctuary is situated to the north-east of the North Market of Miletus, close to the Lion Harbor.
Since the cult of Apollo Delphinios is Cretan in origin, it is assumed that a sanctuary existed at Miletus for the worship of Apollo Delphinios from the period of the earliest settlers. There is literary evidence for a Delphinion at Miletus in the sixth century B.C., Diogenes Laertius 1.29, although the earliest remains at the site of the present Delphinion date to the fifth century B.C., and the location and form of the archaic sanctuary are uncertain.
The earliest preserved remains at the Delphinion are the rectangular altar with volute acroteria, and a number of marble round altars; these predate the Persian destruction of 494 B.C. It is thought that the round altars were collected from various locations and brought to the Delphinion at this time. In the fifth century B.C., when Miletus was rebuilt, the Delphinion took the form of a small rectangular enclosure of ca. 30 x 45 m., which was bordered by stoas at the north and south.
Fragments of archaic building materials were reused. In the late fourth century B.C., the sanctuary was renovated and enlarged, expanding to the east and now measuring ca. 61 x 51 m. New two-aisled stoas with inner and outer colonnades of the Doric order were built at the north, east and south sides, while the west side was closed off with a wall. In the late Hellenistic period, the enclosure was made completely peristylar with the addition of a one-aisled stoa at the west. In the mid-second century A.D., a circular shrine or monopteros was constructed in the temenos, and the porticoes were altered to single-aisled colonnades with Corinthian capitals. A propylon was also erected in the middle of the west side.