The ancient gymnasium of Olympia lies north-west of the Altis enclosure on a flat stretch of land by the Kladeos river bank. It is adjacent to the palaestra, which extends the gymnasium complex towards the south. Here athletes practiced track and field and the pentathlon. Before the construction of the gymnasium in the Hellenistic period, these events took place outdoors. The surviving structure dates to the second century BC.
The gymnasium is a large quadrangular building, with central court enclosed by Doric stoas. A series of rooms for the athletes probably occupied the west wing. The better studied east wing consists of a solid outer wall, an internal double Doric colonnade, and another colonnade of sixty columns along the court. The lower courses of the outer wall were of poros blocks with stone-built buttresses on the exterior, while the upper courses were of brick. The stoa, like the stadium, was one Olympic stade long, and had ruts on the floor marking the starting-point and finishing post, so that the athletes practiced the exact same distance as they would run during the games. The internal colonnade divided the stoa longitudinally into two parallel tracks: the xystos, the floor of which had to be regularly scraped and leveled (xystos=scraped); and, on the side of the court, the paradromis, or auxiliary track. The spacious court, approximately two hundred and twenty metres long and a hundred metres wide, was used to practice the javelin and discus. A monumental propylon was added at the south-east corner of the building, opposite the north-west entrance to the Altis, in the late second century BC. This propylon consisted of a Corinthian portico, 15.50 metres long and 9.80 metres wide, raised on steps. The propylon's interior was divided longitudinally into three naves by two rows of Corinthian columns; the entablature was decorated with bovine heads and supported a coffered stone ceiling. The south stoa, which communicates with the adjacent palaestra to the south, was added in the first century BC.
The gymnasium is only partly preserved. Its west wing was swept away by the Kladeos river, while its north section has not yet been investigated. The surviving remains were excavated and studied by the German School in recent years.