The siege of Thessalonica (676–678 CE) occurred amidst the backdrop of increasing Slavic presence and pressure on the Byzantine Empire. Initial Slavic incursions began during the reign of Justinian I (527–565 CE), escalating with the support of the Avar Khaganate in the 560s, leading to significant settlements in the Balkans. The Byzantine Empire's focus on Eastern conflicts and internal strife facilitated Slavic and Avar advances, culminating in a notable presence around Thessalonica by the 610s, effectively isolating the city.

By the mid-7th century, cohesive Slavic entities, or Sclaviniae, had formed, challenging Byzantine control. The Byzantine response included military campaigns and relocation of Slavs to Asia Minor by Emperor Constans II in 658. Tensions with the Slavs intensified when Perboundos, a Slavic leader, was arrested and later executed by the Byzantines, sparking a revolt. This led to a coordinated siege by Slavic tribes on Thessalonica, exploiting the Byzantine preoccupation with Arab threats.

The siege, characterized by frequent raids and a blockade, strained the city through famine and isolation. Despite the dire situation, miraculous interventions attributed to Saint Demetrius and strategic military and diplomatic responses by the Byzantines, including a relief expedition, eventually alleviated the city's plight. The Slavs continued raids but shifted focus to naval engagements until the Byzantine military, finally able to address the Slavic threat post-Arab conflict, decisively countered the Slavs in Thrace.

Scholarly debate on the siege's precise chronology has varied, with current consensus favoring 676–678 CE, aligned with the First Arab Siege of Constantinople. This period marks a significant episode in the Byzantine-Slavic interactions, highlighting the complexities of medieval Balkan politics and the resilience of Thessalonica amidst external pressures.