In the flat area extending to the Northwest of the Sacred Altis and to the East of the bed of the river Kladeos the Gymnasium an elongated, monumental complex was erected during the Hellenistic period. The Gymnasium is situated to the North of the older Palaestra, both of them constituting the main facilities for the preparation of the athletes prior the competition in the Olympic Games.Τhe Gymnasium was the venue for the training of the athletes of the running events and of the pentathlon. The structure extends on the axis North-South and consists of four porticos surrounding a central open-air courtyard. The porticos were built gradually, while their precise dating is not known. Its length reaches 220 m and its width is 120 m, a size similar to that of the stadion covering a surface that exceeds 26 acres.

Initially, in the early 2nd century B.C., the single aisled south portico of the Gymnasium was erected with an external colonnade facing the central courtyard of the ionic order according to Mallwitz or Doric according to Graef. This portico is built adjacently to the north stoa of the Palaestra in such a manner that the two edifices are sharing actually their back wall and are connected via an opening.

The construction of the east portico will follow in the 2nd century B.C. It is double-aisled, facing the central courtyard to the west, with a length of 212,05 m and a width of 11,50 m. Its external wall is supported at intervals with buttresses, while its lower section is built of blocks of poros and its upper structure is of raw bricks. The outer colonnade was consisted of 60 columns and the inner 66, supported on single bases. Both these markedly araeostyle (3,14 m.) colonnades - a feature implying that the entablature was of wood - were of the Doric order. This portico hosted the xystos, the roofed track where the athletes used to train, when the weather conditions were harsh. Parallel to the xystos was the open-air corridor named paradromis. Along the stylobate of the exterior colonnade a water drainage gutter of poros stone blocks was constructed to collect the rain water.

Probably in the 2nd or 3rd centuries CE the upper structure of bricks was replaced by a mixed masonry of bricks, stones and reused architectural members. From the initial phase of the east stoa the foundations, the lower part of the walls with small sections of the later upper structure, the stylobate and the gutter are preserved, as well as the fluted and unfluted drums of the columns.

The recent excavation of the Ephorate of Antiquities of Ilia has brought to light to quite a large extent the mortar that covered the inner surface of the eastern wall, and also to its west many fallen sections with traces of painted wavy lines, an important piece of evidence that the east wall of the portico bore internally painted decoration.

In the end of the 2nd or the 1st century BCE in the southeast corner of the complex, opposite the entrance to the Sacred Altis, close to the Prytaneion, a particularly elegant monumental aphiprostyle propylon was erected. Ιt is comprised of a stepped krepis which supported eighteen colums of the Corinthian order forming a three-aisled entrance to the complex of Gymnasium. The decoration of the entablature with bucrania and rosettes in relief was particularly well elaborated, while the ceiling bore stone cassettes. In the early Byzantine era it was demolished and some of its members were reused in the former Pheidias? Workshop, which was converted into a church.

From the north portico only a small section has been discovered across the provincial road Olympia-Linaria, while the floods of the Kladeos river in the 4th century CE may have washed away the whole west portico, from which solely traces have been uncovered. Probably it had two storeys and served for the accommodation of the athletes.

The Gymnasium was first investigated by the German Archaological Institute in the years 1878-1880, when the propylon, the south stoa, the southernmost part of the east stoa as well as its northern edge were revealed, whereas in 1936, a section towards the north of the east stoa and part of the north stoa came to light. During the years 2013-2015 the Ephorate of Antiquities of Ilia excavated the east stoa and the courtyard of the monument towards the North, while to the East of the monument a building dating to the Late Antiquity was uncovered. It should be pointed out that this excavation was the first-ever to have been conducted by the Greek Archaeological Service within the core of the Archaeological Site of Olympia.