Compared to the other parts of the site, the Gymnasium was built fairly late, during the Hellenistic Era, and the ruins visible today are what remains of a building dating back to the 2nd century BC.Most of our knowledge regarding its structure comes from the east wing, which has been more extensively excavated and studied, with the last large-scale excavation completed in 2015, while the west wing has been swept away by the river. The building was mainly used by track-and-field and pentathlon athletes, who had previously trained at the same spot, though without the added comfort and protection of a walled structure.
The rectangular Gymnasium was walled all around, and the large court at the centre was the same length as the actual Olympic stadium, so that athletes could run the same distance during training as they would during the Games. A double Doric colonnade divided the east stoa into two separate tracks; the main outer track was called the ksystos (Greek for 'scraped'), as its maintenance consisted of constant scraping and levelling of the dirt, while the inner track was auxiliary, hence described as the paradromis (Greek for 'auxiliary track'). Adjacent to the main building was a large court of 22,000 square metres, used for javelin and discus training, which of course was impossible to do indoors.