Built on an islet in the middle of a river that once ran here, the long-deserted Ilissos Basilica is located to the east of the Olympieion, next to the Fokianos Sports Club.

The fifth-century church is dedicated to Saint Leonides, the third-century bishop of Athens who was put to death along with seven female martyrs. Evidence of Saint Leonides’ martyrdom has been discovered at the basilica’s north wall.

The basilica closely resembles the larger Basilica of Lechaion, also dedicated to Saint Leonides and his seven co-martyrs. A mosaic from the Ilissos Basilica, depicting a stork pecking at a snake, is on display at Athens’ Byzantine Museum.