The Sanctuary of Poseidon at Isthmia was an important cult and athletic centre where the Panhellenic Games, called "the Isthmia", took place every two years in honour of Melicertes-Palaemon or Poseidon. The site was fortified in about 1200 B.C. and ritual festivities were performed from the middle of the 11th century B.C. onwards. The temple of Poseidon was first built in the 7th century B.C., and was reconstructed three times from the middle of the 7th century B.C. to 146 B.C. The first Stadium was built during the 6th century B.C., while the peribolos of the shrine of Palaemon and the Theatre were added in the 5th century B.C. During the Roman period, the temple and the theatre were restored while baths and the circular temple of Palaemon were constructed. The sanctuary was abandoned at the end of the 4th century A.D., and later, building material from it was used in the construction of the Hexamilion wall.
The first, restricted excavations, poor in results, were carried out in 1883 by Paul Monceaux and in 1930, by B.S. Jenkins and H. Megaw. Extensive excavations on the site were undertaken by the American School of Classical Studies. During the years 1959-1967, Oscar Brooner excavated the temple of Poseidon, porticoes, the sancuary of Palaemon, the two Stadiums and a Hellenistic settlement at "Rachi", while later, E. Gebhard brought to light the theatre. During 1967-1976 , P. Clement excavated the Roman baths and other buildings. Finally, in 1980 and 1989, E. Gebhard investigated the central shrine and the prehistoric settlement at "Rachi".