One of the most important of the Early Christian basilicas in the Greek world, dedicated to the martyr Leonidas. It was founded around the middle of the 5th century, and completed by the end of the 5th-beginning of the 6th century. It was devastated in the earthquake of 551-552. It is triple-aisle with transverse aisle forming a T. and a with total length, including the two continuous atria, of 179 m. Above the sanctum (bema) rose a square tower with a pyramidal roof supported by heavy double pillars and columns.

The church was bordered by supplementary buildings.At the NW there was a complex with baptistry, probably older than the basilica itself, with a rectangular forecourt, tetra-apsidal vestiary and octagonal baptistry. Excavations near the basilica of Lechaion have revealed the remains of houses of the 6th century A.C., dating both before and after the destruction of the church.

The site was excavated by D. Pallas during the years 1956, 1958, 1959, 1960, 1961 and 1965.