Isthmia (sanctuary of Poseidon), a Corinthian Panhellenic shrine 16 km. (10 mi.) east of Corinth, beside the modern Athens–Corinth road. A hippodrome and hero shrine (West Foundation) lie 2 km. (1¼ mi.) south-west, with additional cults in the Sacred Glen.
The sanctuary was established c.1050 bce in an area of Mycenaean settlement. The first temple (a peripteral i.e. colonnaded building with wall-paintings), c.690–650, had a 30-metre (100-foot) altar and temenos wall. It was rebuilt after fires in c.470–460 and 390. The first stadium (early 6th cent. bce) accords with C. Iulius Solinus' (7. 14) foundation date for the Isthmian Games; a larger stadium (further south-east) was built c.300 bce. A bath (originating c.4th cent.) survives in Roman form. A theatre (established by 390) probably held musical rather than dramatic contests. Isthmia was a major assembly place; it was at the games in 196 bce that T. Quinctius Flamininus announced the freedom of the Greeks (see freedom in the ancient world).
After L. Mummius sacked Corinth in 146 bce, Isthmia was abandoned and the games transferred to Sicyon; returned to the Roman colony c.2 bce, they were resumed at Isthmia c.50–60 ce, when the theatre was renovated, followed, by 100, by the temple and temenos wall. A heroon of Palaemon (see melicertes) dates from the mid-1st cent. ce; the first, Hadrianic, temple of Palaemon was transferred during the reign of Marcus Aurelius. Cult activity ceased during the 3rd cent.
On the Rachi ridge to the south is a small, mainly Hellenistic, settlement (abandoned c.200 bce).