Corinth remained a thriving city in Late Antiquity, on account of its strategic location and the commercial activity in its two seaports, Kechrees and Lechaion. Earthquakes in 365 and 375 dealt the city a serious blow, as did Gothic raids in 395/6. City space was confined within the newly built Late Roman or Early Byzantine walls, parts of which still survive in places in the village of Ancient Corinth. Though previously linked by walls to Acrocorinth, the city now lost this connection. The significance of the ancient forum seems to have diminished, and archaeological evidence points to the domestic activities and small industries of a poor neighbourhood.

The Early Byzantine period (especially the 5th and 6th century) saw the building of numerous large Christian basilicas with substantial sculpture decoration in Corinth and its environs, such those at Kraneion to the east, Kodratos to the north and Skoutela to the northeast, and those at Lechaion and Kechrees. Cemeteries dating to the 6^th and 7^th century have been excavated in areas outside the walls, at the Asclepeion and the basilicas at Kodratos and Kraneion. The city’s recovery in this period seems to have been stalled by the great famine of 542, which affected demographics, and a devastating earthquake in 525 mentioned by Procopius .

The building of the Hexamilion wall was of particular importance in protecting both the city and the entire Peloponnese. The wall ran the length of the isthmus, from the shores of the Saronic Gulf to the Gulf of Corinth, rendering it one of the largest fortification projects. The wall was originally built by Emperor Theodosius II (408-451), and rebuilt by Justinian between 548 and 560 in the wake of devastating earthquakes in the first half of the 6th century.

Over the following centuries there are signs of activity around the basilicas outside the walls; from the late 6^th to the 8^th century the Roman forum was used as a burial ground.

Excavation finds from the basilicas at Kraneion, Kodratos and a small basilica at Acrocorinth reveal signs of habitation in the 7th century. Similar activity has also been detected at Diabatiki near Lechaion. In particular, building remains and other finds from the Kraneion area show that it was inhabited in the mid and late-Byzantine periods.

After the empire was reformed in the late 8th century, we know that Corinth became the capital of the Theme of the Peloponnese, and the seat of a general. As an archdiocesal see it must have had a large cathedral. Despite the lack of excavation data, a large number of sculptures dating from the 9th to the late 12th-early 13th century indicate the existence of churches already known from sources, such as those dedicated to Agioi Theodoroi and Christ the Saviour, and the Latin-rite monastery of St. Nicholas. There are some remains of the small basilica erected on the site of the Roman "bema" where Paul preached; excavations have also identified churches at the Peirini spring and to the south of the museum. Added to these are the church of Agios Ioannis (St. John the Theologian), which survived until 1937, and the now ruined church of Agia Paraskevi, etc.

As Corinth’s location favoured the development of trade, the city probably became an important centre in the Middle Byzantine period. Coins and treasures dating from this period attest to economic growth in the city. In the late 11^th century the Venetians used Corinth to gather local products such as silks and olive oil, while in 1165-1171 Vitale Voltani, a representative of Romano Raimano, monopolized the Corinthian olive oil market on Venice’s behalf. The city was also famed for its trade in currants, derived from the renowned local variety of grape. Archaeological evidence shows that in the late 11th century various buildings encroached on the open space of the Roman forum.Though building phases are unclear, these included shops, housing and bath complexes, monasteries and a number of workshops. There is firm archaeological evidence of workshops for ceramics, glass, gold and brass. Silk has not been found, despite clear textual references to installations for processing and dyeing it.

In 1147 Corinth was raided by a fleet sent by Roger II of Sicily. The city retained its commanding presence nonetheless; Roger’s geographer al-Idrisi describes it as large and prosperous, while in the late 12th century Choniates mentions the two ports of Lechaino and Kechrees, and the bustling commercial activity below Acrocorinthos castle. Excavation evidence indicates that the city must have been fortified with towers and a perimeter wall when the Franks arrived in the 13th century. Trading activity flourished despite the change in administration. The remains of a contemporary neighbourhood to the west of the Roman forum indicate that large numbers of clay vessels were imported from Apulia and Veneto.

Corinth fell into decline following Catalan raids in 1312, an earthquake in about 1320 and the great plague of 1348. According to a description by Niccolò da Martoni, in 1395 there were only a few dozen houses within Acrocorinthos, while the lower city stood in ruins.