The building is dated to the 1st-2nd centuries A.D.; it had been built on purpose to host music competitions. The same building was also the seat of the Gerousia (Senate), a local authority without considerable power; its main duty was to pay honors to important personalities of ancient Kos.
The cavea («koilon») had fourteen rows of seats made of marble; only nine of them have been restored. A passage divided the theater in an upper and a lower diazoma; four stairways divided the lower part in cunei. The stage was of pentagonal shape with a proscenium and wings. The orchestra was circular, decorated with marble. Two entrances, decorated with nice mosaic floors, were found at the ends of the stage. Several statues, situated in niches, decorated the arcades. Some of them were brought to light during the excavations of Italian archaeologists, carried out in 1929. The theater was restored in 1934, also by Italians.