The Hexamilion wall (Greek: Εξαμίλιον τείχος, "six-mile wall") was a defensive wall constructed across the Isthmus of Corinth, guarding the only land route onto the Peloponnese peninsula from mainland Greece. It was constructed between AD 408 and 450, under the reign of Theodosius II.

The Hexamilion stands at the most recent end of a long series of attempts to fortify the isthmus stretching back to perhaps the Mycenaean period. Many of the Peloponnesian cities wanted to pull back and fortify the isthmus instead of making a stand at Thermopylae when Xerxes invaded in 480 BC (Herodotus' Histories 7.206). The issue arose again before the Battle of Salamis (Herodotos 8.40, 49, 56). Although the concept of a "Fortress Peloponnese" had been repeatedly suggested, fortification of the isthmus was of no utility without control of the sea, as Herodotus notes (7.138).

The wall was constructed from AD 408 and 450, in the reign of Theodosius II during the time of the great Barbarian invasions into the Roman Empire. Its purpose was to protect the Peloponnese from invasion from the north. The attack of Alaric on Greece in 396 or the sack of Rome in 410 by the Visigoths may have motivated its construction. The wall ran from the Gulf of Corinth to the Saronic Gulf, covering a distance of 7,028 and 7,760 meters. The fortress contained two gates (north and south), of which the northern gate functioned as the formal entrance to the Peloponnese. In the reign of Justinian, the wall was fortified with additional towers, reaching a total number of 153, with forts at either end and the construction of Justinian's Fortress at Isthmia. The building of the Fortress at Isthmia was left mostly to autonomous work crews that, while following the same general instructions and using the same materials, operated in markedly different ways. As for the wall itself, local Corinthians – irrespective of politics or religion – would have contributed to the physical construction of the Hexamilion and the maintenance of any associated garrisons. Military use appears to have fallen off after the 7th century, and by the 11th century domestic structures were being built into the wall.

The strategic fortress of Isthmia, taking advantage of favorable terrain, was located to the southern side of the Hexamilion wall, north-east of the Poseidon Sanctuary. The wall was constructed with a rubble and mortar core faced with squared stones. The blocks on the northern facade were larger and coalesced with more carefully implemented edges, while the southern face was conceived of smaller stones set in mortar. It is not certain how long it took to complete, but the importance given to the task is apparent from the scale of the construction; the Hexamilion is the largest archaeological structure in Greece. Due to the great mass of the 7.5 km long wall (which was 7m high and 3m thick) many structures in the region were cannibalized for stone for the effort. Some structures were incorporated into the wall directly (as was the temple of Poseidon at Isthmia) whereas some were burned into lime (as was the sanctuary of Hera at Perachora, as well as much of the ancient statuary of Corinth). Materials from the Sanctuary of Poseidon were evenly distributed and converted into the main entrance of the wall in an emplecton building technique in the first century. Spolia (voussoirs, column drums, and inscribed blocks) were incorporated into both the structure and roadway. The fortress was intimately tied into the defensive network, a fact readily demonstrated by similarities in construction techniques used. The fortress consisted of nineteen rectangular towers protruding from the walls of its 2.7-hectare total area, and more than likely housed the military garrison that defended the Hexamilion as a whole. The main passageway through the wall was through the Isthmia fortress, where the north-east gate acted as the main entrance into the Peloponnese.

It is likely that the fortifications were damaged severely by earthquakes, which contributed to the rapid deterioration of the wall between renovations during Justinian and Manuel II's reigns. Most damaging was perhaps the earthquake of 551, which Procopius mentions as being particularly destructive to Greece as a whole.

The garrison of the fortress of Isthmia in the 5th century likely consisted of four to eight tagmata. Historians believe the quality and state of the troops were similar to that of Procopius' descriptions of the state of the soldiers that manned the fortification at Thermopylae prior to Justinian’s reign; namely, local farmers who proved to be incapable of checking the advance of various invaders and so were replaced by comitatenses. As part of his repairs to the wall, Justinian established a professional military garrison within the Fortress of Isthmia, which replaced the local farmers who previously manned it.

To bolster supplies, the soldiers produced some of their own food through farming south of the Hexamilion, although major aid came also from local farmers, merchants, artisans, and workmen, including from other nearby towns, such as Corinth. A system of rural villas supplied a considerable share of goods and services also; such villae rusticae being an important part of the economic exchange system of the Empire, and a basic productive unit of Late Roman and Early Byzantine times. The variety of skilled labor contributed by the Hexamilion garrison allowed for the creation of local granaries, allowing for intensified economic exploitation of the region. Despite this growth in developmental pace, the demands on the countryside and local economy fluctuated seasonally, with a notable intensification of economic activity during the warmer seasons. Likewise, the garrison's presence strained both the environment and local economy during the off-season, when their skills were not in use. This created a cyclical local economy based on the presence of troops, where demand and production were in constant flux.

During its initial construction, the Hexamilion significantly restricted the number of passages into the Peloponnese. The road from Athens was made to pass directly through the eastern fortress towards Corinth to the west and Epidaurus to the east. This transformed the fortress of Isthmia and its attendant wall section into the main overland connection to southern Greece. The wall's guarded gateways allowed for taxation of incoming and outgoing trade, which helped boost the local economy of the region. The Hexamilion wall likely had both short and long-term negative effects on the local population as well. The acquisition of land and clearing of buildings along the route of the wall led to conflict with individual property holders. In addition to its defensive role, the wall likely functioned as a means to entrench state control over local affairs. While the scale of the repairs on the Hexamilion wall during Justinian's reign suggests the fortification project would have provided employment to local laborers, which influenced the distribution of wealth within the local economy, and likely attracted many skilled laborers to the region.