Caesaropapism /ˌsiːzəroʊˈpeɪpɪzəm/ is the idea of combining the social and political power of secular government with religious power, or of making secular authority superior to the spiritual authority of the Church, especially concerning the connection of the Church with government. Although Justus Henning Böhmer (1674–1749) may have originally coined the term caesaropapism (Cäseropapismus), it was Max Weber (1864–1920) who wrote that "a secular, caesaropapist ruler ... exercises supreme authority in ecclesiastic matters by virtue of his autonomous legitimacy." According to Weber, caesaropapism entails "the complete subordination of priests to secular power."

In an extreme form, caesaropapism is where the head of state, notably the emperor ("Caesar", by extension a "superior" king), is also the supreme head of the church (pope or analogous religious leader). In this form, caesaropapism inverts theocracy (or hierocracy in Weber), in which institutions of the church control the state. Both caesaropapism and theocracy are systems in which there is no separation of church and state and in which the two form parts of a single power-structure.

Caesaropapism's chief example is the authority that the Byzantine (East Roman) emperors had over the Church of Constantinople and Eastern Christianity from the 330 consecration of Constantinople through the tenth century. The Byzantine emperor would typically protect the Eastern Church and manage its administration by presiding over ecumenical councils and appointing Patriarchs and setting territorial boundaries for their jurisdiction. The emperor exercised a strong control over the ecclesiastical hierarchy, and the Patriarch of Constantinople could not hold office if he did not have the emperor's approval. Such emperors as Basiliscus, Zeno, Justinian I, Heraclius, and Constans II published several strictly ecclesiastical edicts either on their own without the mediation of church councils, or they exercised their own political influence on the councils to issue the edicts. According to Metropolitan Kallistos Ware, the historical reality of caesaropapism stems from the confusion of the Byzantine Empire with the Kingdom of God and the zeal of the Byzantines "to establish here on earth a living icon of God's government in heaven."

However, Caesaropapism "never became an accepted principle in Byzantium." Several Eastern churchmen such as John Chrysostom, Patriarch of Constantinople and Athanasius, Patriarch of Alexandria, strongly opposed imperial control over the Church, as did Western theologians like Hilary of Poitiers and Hosius, Bishop of Córdoba. Saints, such as Maximus the Confessor, resisted the imperial power as a consequence of their witness to orthodoxy. In addition, at several occasions imperial decrees had to be withdrawn as the people of the Church, both lay people, monks and priests, refused to accept inventions at variance with the Church's customs and beliefs. These events show that power over the Church really was in the hands of the Church itself – not solely with the emperor.

During a speech at the St. Procopius Unionistic Congress in 1959, John Dvornik stated, "...the attitude of all Orthodox Churches toward the State, especially the Russian Church is dictated by a very old tradition which has its roots in early Christian political philosophy... the Christian Emperor was regarded as the representative of God in the Christian commonwealth, whose duty was to watch not only over the material, but also the spiritual welfare of his Christian subjects. Because of that, his interference in Church affairs was regarded as his duty." The regional church was elevated by the rivals of the Byzantine Empire, namely the Serbian and Bulgarian empires, to patriarchate according to a prevailing theory during the time the status of the church had to be equal to the state.

Following the Fall of Constantinople in 1453, the sultans of the Ottoman Empire took control of appointing the Patriarch of Constantinople and all Byzantine Rite Bishops within their dominions. According to historian Charles A. Frazee, the Greek Hierarchs appointed by the sultan and his advisors were almost invariably opposed to the reunification decrees at the Council of Florence and rejected the authority of the Papacy.

At the same time, however, so great was the suffering of the Greek people under the Sultans that, in the February 14, 1908 Papal allocution Ringraziamo Vivamente, Pope Pius X accused the Greek Orthodox Church under Turkish rule of having preferred, "a harsh yoke (that of Islam) to the tenderness of their mother."

Caesaropapism was most notorious in the Tsardom of Russia when Ivan IV the Terrible assumed the title Tsar in 1547 and subordinated the Russian Orthodox Church to the state. In defiance of the Tsar's absolute power, Philip, the Metropolitan of Moscow, preached sermons in Tsar Ivan's presence that condemned his indiscriminate use of state terror against real and imagined traitors and their families by the Oprichnina. Metropolitan Philip also withheld the traditional blessing of the Tsar during the Divine Liturgy. In response, the Tsar convened a Church Council, whose bishops obediently declared Metropolitan Philip deposed on false charges of moral offenses and imprisoned him in a monastery. When the former Metropolitan refused a request from the Tsar to bless the 1570 Massacre of Novgorod, the Tsar allegedly sent Malyuta Skuratov to smother the former Bishop inside his cell. Metropolitan Philip was canonized in 1636 and is still commemorated within the Orthodox Church as a, "pillar of orthodoxy, fighter for the truth, shepherd who laid down his life for his flock."