The square dates back to ancient Byzantium, before its conversion into an imperial capital by Constantine the Great. When Roman Emperor Septimius Severus (r. 193–211) rebuilt the city, he erected a large square surrounded by porticoes, hence named the Tetrastoon ("four stoas"). In the center of the square stood a column with a statue of the god Helios. In the 320s, Constantine adorned his chosen new capital with many new monumental buildings. His activities included new structures around the Tetrastoon, while the Augustaion was likely carved out of its eastern part at that time, and named after a Porphyry column supporting a statue of his mother, the Augusta Helena. The Augustaion was rebuilt in 459 under Emperor Leo I (r. 457–474), and again in the 530s, after being destroyed in the Nika riot, by Emperor Justinian I (r. 527–565). In its original form, the square was open to the public and functioned as the city's food market (agora), but after Justinian's reconstruction, it became more of an enclosed courtyard where access was restricted. Byzantine writers from the 7th century on refer to it as explicitly as a court or forecourt (αὐλή, αὐλαία, προαύλιον) of the Hagia Sophia.
Justinian's Augustaion survived mostly unchanged through the subsequent centuries. In the late 13th century, following the recovery of the city from the Latin Empire, the square and its adjacent buildings seem to have been the property of the Hagia Sophia. By the early 15th century however, the Italian traveller Cristoforo Buondelmonti reported that the square lay in ruins, and by the time of Pierre Gilles' sojourn in the 1540s, only the fragments of seven columns remained.