Poseidon Temple of Isthmia, Ancient Greek temple in Isthmia, Greece.

The Poseidon Temple of Isthmia is an ancient Greek sanctuary on the Isthmus of Corinth, about 16 km east of ancient Corinth. The Archaic temple, built between 690 and 650 BC, had a long plan of about 40 by 14 m with a wooden peristyle; it was destroyed by fire in 470 BC. Its Classical successor, rebuilt around 440 BC, had six columns across the front and thirteen along the sides, local stone walls, and a marble roof. Today low courses of small grey and tan stones trace the rectangular foundations, with loose squared blocks nearby.

The earliest temple was built during the Archaic period but was destroyed by fire and rebuilt in the Classical era. This reconstruction made the site one of the region's most important religious centers.

The sanctuary hosted the Isthmian Games, one of the four Panhellenic festivals, held in Poseidon's honour from around 581 BC. Pausanias noted bronze Tritones on the temple and a temple of Palaimon, where oaths were sworn; the Isthmian Games were reorganized in the 47th Olympiad (584–580 BC).

The site is generally open during daylight hours with viewing areas for the ancient structures and surrounding grounds. Shade is limited at the archaeological site, so it is wise to bring water and sun protection.

Archaeologists uncovered objects from nine different Greek city-states at this location, showing how far pilgrims traveled to visit. These discoveries revealed a network of trade routes and religious exchange that connected the ancient Mediterranean world.