This triumphal arch was erected in the 2^nd century AD in honor of the Roman emperor Hadrian. The arch stands within walking distance of Syntagma Square. It was probably constructed before Hadrian's visit to Athens on 131/132AD. The arch was constructed spanning an ancient road which led from the ancient city center where the Agra and Acropolis stand to the Olympieion and southeast Athens. The western side of the arch bears the inscription: "This is Athens, the ancient/old city of Theseus." On the eastern side an inscription states: "This is the city of Hadrian and not of Theseus." The inscriptions could signify that the arch was built on the line which divided old Athens to the west from Hadrian's new Athens (Hadrianoupolis) to the southeast. The inscriptions could also be interpreted as proclaiming Hadrian as the new hero of all of Athens, replacing Theseus.

The Pentelic marble arch stands 59 feet high, is 41 feet wide and 8 feet deep. On top of the arch opening are pilasters of Corinthian rhythm. The arch or gate way was not originally part of a wall but during the Turkish occupation in the 18^th century it was incorporated as one of the seven gates in the defensive walls against Albanian invaders.