The Byzantine Iconoclasm (Greek: Εικονομαχία, romanized: Eikonomachía, lit. 'image struggle', 'war on icons') were two periods in the history of the Byzantine Empire when the use of religious images or icons was opposed by religious and imperial authorities within the Orthodox Church and the temporal imperial hierarchy. The First Iconoclasm, as it is sometimes called, occurred between about 726 and 787, while the Second Iconoclasm occurred between 814 and 842. According to the traditional view, Byzantine Iconoclasm was started by a ban on religious images promulgated by the Byzantine Emperor Leo III the Isaurian, and continued under his successors. It was accompanied by widespread destruction of religious images and persecution of supporters of the veneration of images. The Papacy remained firmly in support of the use of religious images throughout the period, and the whole episode widened the growing divergence between the Byzantine and Carolingian traditions in what was still a unified European Church, as well as facilitating the reduction or removal of Byzantine political control over parts of the Italian Peninsula.

Iconoclasm is the deliberate destruction within a culture of the culture's own religious images and other symbols or monuments, usually for religious or political motives. People who engage in or support iconoclasm are called iconoclasts, Greek for "breakers of icons" (εἰκονοκλάσται), a term that has come to be applied figuratively to any person who breaks or disdains established dogmata or conventions. Conversely, people who revere or venerate religious images are derisively called "iconolaters" (εἰκονολάτρες). They are normally known as "iconodules" (εἰκονόδουλοι), or "iconophiles" (εἰκονόφιλοι). These terms were, however, not a part of the Byzantine debate over images. They have been brought into common usage by modern historians (from the seventeenth century) and their application to Byzantium increased considerably in the late twentieth century. The Byzantine term for the debate over religious imagery, "iconomachy," means "struggle over images" or "image struggle". Some sources also say that the Iconoclasts were against intercession to the saints and denied the usage of relics, however it is disputed.

Iconoclasm has generally been motivated theologically by an Old Covenant interpretation of the Ten Commandments, which forbade the making and worshipping of "graven images" (Exodus 20:4, Deuteronomy 5:8, see also Biblical law in Christianity). The two periods of iconoclasm in the Byzantine Empire during the 8th and 9th centuries made use of this theological theme in discussions over the propriety of images of holy figures, including Christ, the Virgin (or Theotokos) and saints. It was a debate triggered by changes in Orthodox worship, which were themselves generated by the major social and political upheavals of the seventh century for the Byzantine Empire.

Traditional explanations for Byzantine iconoclasm have sometimes focused on the importance of Islamic prohibitions against images influencing Byzantine thought. According to Arnold J. Toynbee, for example, it was the prestige of Islamic military successes in the 7th and 8th centuries that motivated Byzantine Christians to adopt the Islamic position of rejecting and destroying devotional and liturgical images. The role of women and monks in supporting the veneration of images has also been asserted. Social and class-based arguments have been put forward, such as that iconoclasm created political and economic divisions in Byzantine society; that it was generally supported by the Eastern, poorer, non-Greek peoples of the Empire who had to constantly deal with Arab raids. On the other hand, the wealthier Greeks of Constantinople and also the peoples of the Balkan and Italian provinces strongly opposed Iconoclasm. Re-evaluation of the written and material evidence relating to the period of Byzantine Iconoclasm has challenged many of the basic assumptions and factual assertions of the traditional account. Byzantine iconoclasm influenced the later Protestant reformation.