To the west of the market arcade stands a large fountain structure, a nymphaeum, that is over 40 meters long. It was supplied with water from the Klepsydra spring, located within the village of Mavrommati. Its location next to the large northern arcade of the market allows us to identify it with the Arsinoe Fountain mentioned by Pausanias (fig.). It consists of an elongated reservoir approximately 40 m long which is situated a short distance in front of a tall, pseudo-isodomic abutment featuring an opening on its right side with a water conduit for filling the reservoir. Between the cistern and the abutment was a shallow colonnade of Ionic half-columns. A semicircular pedestal (platform) located exactly in the center of the upper cistern supported a group of bronze statues. Two additional basins are located on a slightly lower level than the first, symmetrically arranged on either side of a paved atrium. The façade of the fountain’s first construction phase (3rd century BCE) was topped by a Doric colonnade, which was removed during the second phase in the 1st century CE.

The second phase of restoration and modification of the fountain’s form is dated to the reign of Emperor Nero, and the theme of the composition (bronze statues of the Augustan family) displayed on the semicircular platform. The third and final phase of repairs and alterations to the Fountain, which included the addition of the two square projections protruding symmetrically from the ends of the front side, dates to the late 3rd century AD. The dating is based on a bronze coin of Emperor Diocletian (284–305 AD), found beneath the paving of the west wing.

The fountain shared the fate of the city’s other buildings, which were abandoned around 365–370 A.D. This was a consequence of the broader economic collapse of the Roman Empire, which had begun earlier, and the final disintegration accelerated by barbarian raids and earthquakes. The eastern section of the Fountain remained standing and was used through the early Christian period, as evidenced by the structures in the upper reservoir and the building (watermill) on the lower level in front of the Fountain, constructed entirely of spolia.