The ancient city of Constantinople was divided into 14 administrative regions (Latin: regiones, Greek: συνοικιες, romanized: synoikies). The system of fourteen regiones was modelled on the fourteen regiones of Rome, a system introduced by the first Roman emperor Augustus in the 1st century AD.

After Emperor Constantine the Great re-founded Byzantium as Constantinople and Nova Roma ('New Rome') in the early 4th century, he or his immediate successors divided Constantinople into its own 14 regiones. Each region (regio) was numbered, and their boundaries and landmarks in the 5th century were enumerated by the Notitia Urbis Constantinopolitanae, which also gives details of the city's Cura Annonae, the public grain ration which was distributed by regio. Two regiones, XIII and XIV, were outside the original city walls.

The area of the I^st regio was defined by the Great Palace, which lay within it, the southeastern edge of Hippodrome, the Byzantine acropolis, and the sea (the Propontis). Here, besides the Great Palace, there were a number of palatial buildings named by the Notitia Urbis Constantinopolitanae: there was the Palace of Placidia, (Palatium Placidianum) connected with a daughter of Valentinian I, and a House of Galla Placidia (Domus Placidiae Augustae), associated with the augusta Galla Placidia, daughter of Theodosius I and husband of first Athaulf and then Constantius III. A House of the Nobilissima Marina (Domum nobilissimae Marinae) was probably named for Marina, a daughter of Galla Placidia's brother Arcadius augustus, who himself gave his name to the Baths of Arcadius, (Thermas Arcadianas) described by Procopius.

The II^nd regio was the site of Hagia Sophia and Hagia Irene, which the Notitia calls the Magna Ecclesia, 'Great Church' and the Ecclesia Antiqua, 'Old Church', a court-house, and the huge Baths of Zeuxippus. These thermae were on the southern side of the Augustaion square and extended as far as the northern edge of the Hippodrome and the Great Palace. On the eastern edge of the Augustaion beside Hagia Sophia was a senate-house for the Byzantine Senate, (to be distinguished from the senate-house in the Forum of Constantine) which is mentioned in the Chronicon Paschale and by Procopius, who called it a bouleuterion (Ancient Greek: βουλευτήριον). Though the II^nd regio encompassed buildings on three of the Augustaion's four sides, the square itself was counted in the IV^th regio. East of the Augustaion and downhill from Hagia Sophia and Hagia Irene was the classical theatre (Latin: Theatrum Minus, lit. 'lesser theatre'). Near the old Byzantine acropolis was the amphitheatre (Latin: Theatrum Maius, lit. 'greater theatre'), known also as the Cynegion (Ancient Greek: Κυνήγιον) and which was probably eastward of the acropolis, near the site of the kitchens of the Topkapı Palace. Both theatre and amphitheatre pre-existed Constantine's re-foundation of the city.

The III^rd regio was largely filled with the large circus, called the Hippodrome, which was Constantinople's equivalent of the Circus Maximus in Rome. Its southern edge was the harbour that Zosimus called the "harbour of Julian", otherwise known as the Kontoskalion. Its northern limit was marked by the first stretch of the Mese odos - the main city thoroughfare running east–west - from its origin at the Milion to the Forum of Constantine. It apparently included a tribune on the southern edge of that forum, which itself lay at the junction of regiones VI, VII, and VIII. There was also the semi-circular colonnaded area known as the Sigma, which Zosimus likewise attributed to Julian. The Cistern of Philoxenos was located there, as was the later Justinianic Church of Saints Sergius and Bacchus, and a residence named for the augusta Pulcheria (Domum Pulcheriae Augustae).

The IV^th regio was the site of the Milion (Μίλλιον), the terminus of the Mese odos and, in emulation of Rome's own Milliarium Aureum ('Golden Milestone'), the milestone to which all road distances in the empire were measured. The Milion was a quadrifrons archway. The IV^th regio ranged from the Augustaion, which it included, along the valley northwards to the Golden Horn. On the west-northwestern edge of the Augustaion was a great basilica complex, possibly of Severan date, under which was a huge Justinianic water-storage facility that still survives: the Basilica Cistern. The arrangement resembles the layout of the basilica, forum, tetrapylon, and colonnades at Leptis Magna, which suggests the orientation and basic appearance of this part of the city was determined before Constantine by the building works of the Severan emperors. Near the senate-house of the Augustaion was a sculpted marble galley, a victory monument commemorating a sea-battle. To the north of the acropolis, near the Prosphorion Harbour in the V^th regio, as was the stadium, also likely a pre-Constantinian building and no longer used for sports when Procopius alluded to it in the 6th century; Justinian had some guesthouses constructed in its vicinity. The Notitia also mentions a church or martyrium to Saint Menas, and the Scala Timasii, a quay apparently named for the 4th century general Timasius, victor of the Battle of the Frigidus.

The V^th regio was a commercial district bounded by the Golden Horn to the north and the Mese odos to the south. The Notitia says of it in the early 5th century: "in this Region are contained the buildings that supply the city with its necessities." The Prosphorion Harbour (Portus Prosphorianus) - the 'import harbour' - was sited there. There were four horrea: the olive oil warehouses (horrea olearia) the Horrea Troadensia, 'Troadensian warehouses', Horrea Valentiaca, 'warehouses of Valens', and the Horrea Constantiaca, 'warehouses of Constantius'. Additionally, there was the Prytaneum and the Strategium, which probably were at the civic heart of old Byzantium. The strategion was probably the agora of the classical city, likely named for the official seat of the strategoi there. In Constantine's city, the Strategium was large area; Theodosius I was able to build a forum complex with a "Theban obelisk" (obeliscus Thebaeus quadrus) within it and leave land to spare in which a market continued to be held; this came to be known as the Lesser Strategion. The Cistern of Theodosius was nearby, and were two thermae named for members of the Theodosian dynasty: the Baths of Honorius and the Baths of Eudocia. The Baths of Eudocia in the Notitia may have been the renamed Baths of Achilles after the marriage of Eudocia to Theodosius II in 421. In the V^th regio too was the ultimate terminus of the Via Egnatia, for the Chalcedonian quay was located here, from which ferries transported traffic to Chalcedon to join the road to Nicomedia and the Asian provinces.

On the shore of the VI^th regio were two maritime facilities on the Golden Horn, alongside the V^th regio's Prosphorion Harbour. These were the portus - the military harbour - and the naval dockyard, the Neorion Harbour, where modern Bahçekapı is. Both the portus and the Neorion were within the walls of the pre-Constantinian city, since Cassius Dio mentions them in his account of the 3rd century siege of Byzantium by Septimius Severus. Another quay, the Scala Sycena, 'Sycaean stairs', served ferries across the Golden Horn to Sycae (later Galata) in the XIII^th regio. In the south of the VI^th regio the boundary was marked by the course of the Mese odos until the round Forum of Constantine, much of which was counted as within the VI^th regio, including the Constantinian senate-house of the Byzantine Senate on the forum's northern axis and the Column of Constantine at its centre.