With the partition of the Roman Empire and the rise of Byzantium in the 4th century AD, the Graeco-Roman world underwent major economic, social, cultural and religious changes. Whereas northern Greece experienced numerous waves of invasions by Slavic and Germanic peoples, Euboea seems to have been spared. Because of its strategic position between the Aegean Sea and continental Greece, the island constitutes a genuine intersection. The fertility of its plains (especially the Lelantine plain), its timber resources in the north (important for naval construction), as well as the abundance of shellfish (indispensable for the dye works of Chalkis and Thebes) all gave it a significant economic role. While earlier Euboean centers shifted or disappeared, like Eretria in the 6th century, others endured, such as Chalkis, where the Emperor Justinian (527-565 AD) restored the fortifications and the bridge over the Euripos in order to control the sea lane of the Euboean Gulf.
After the reorganization of the Byzantine Empire, Euboea became part -under the reign of Justinian II (685-695), at any rate- of the province of Hellas, whose capital was Thebes. Chalkis then served as a harbor for its fleet.
On the ecclesiastical level, Euboea had five dioceses: Euripos (Chalkis), Oreos, Karystos, Porthmos (the port of modern-day Aliverion) and Aulon (today Avlonari). The numerous monasteries and churches built between the 4th and the 12th centuries, such as Aghios Georgios Arma and Aghios Nikolaos above Amarynthos, testify to the wealth of Byzantine culture. During the period of Latin domination, Euboea was a major papal fief, notably from the 14th century to the Ottoman conquest, since the Latin patriarch of Constantinople resided in Chalkis.