The most impressive monument of ancient Eretria, one of the oldest known theatres, lies in the western section of town, between the western gate, the stadium and the upper gymnasium; the temple of Dionysos was found at its south-west end. As indicated by the architectural remains of the scene, the initial construction phase followed the invasion by the Persians and the reconstruction of the city in the fifth century BC, whereas the fourth century BC marked the site's peak.

A striking fact is the construction of the cavea (gr.: koilo, auditorium) on an artificial hill surrounded by numerous retaining walls, instead of taking advantage of the citadel's slopes. During the first building phase, the scene looked like a palace, disposed of five adjacent rectangle rooms and found itself at the same level as the circular orchestra, leading to it via three entrances. At its peak (fourth century BC), the theatre suffered transformations and was shaped to a large extent in its present form. The cavea comprised eleven tiers divided by ten staircases. The circular orchestra was transferred for 8m to the north, and was lowered by 3m. The scene was amplified by two backstages connected through a portico with an Ionic fa?ade, thus raising above the orchestra. This difference in heights was evened up by a vaulted underground gallery, leading through the scene to the centre of the orchestra; this was in all probability the ?charonian stairway? (stairs of Hades) allowing actors impersonating chthonic deities and the deads to appear and perform at the orchestra. Local poros stone was used for the foundation and limestone for the parodoi (passageways), which sloped to the orchestra in order to diminish the difference in height with the cavea. The theatre seated 6,300 spectators and is reminiscent in form to the Theatre of Dionysos in Athens, after transformation of the latter in 330 BC. Following the destruction of Eretria by the Romans in 198 BC, it was rebuilt with lower quality materials and the rooms to the south of the parodos were then apparently decorated with colour mortars of the first Pompeian style.

Unfortunately, most benches have been looted. There are still the impressive remains of the scene, especially the vaulted underground passage leading to the orchestra centre. Excavation of the monument was undertaken by the American Archaeological School, while the local Ephorate of Antiquities strived greatly for its restoration.