Michael III (Ancient Greek: Μιχαήλ, romanized: Michaḗl; 9/10 January 840 – 24 September 867), also known as Michael the Drunkard, was Byzantine emperor from 842 to 867. Michael III was the third and traditionally last member of the Amorian (or Phrygian) dynasty. He was given the disparaging epithet the Drunkard (ὁ Μέθυσος) by the hostile historians of the succeeding Macedonian dynasty, but modern historical research has rehabilitated his reputation to some extent, demonstrating the vital role his reign played in the resurgence of Byzantine power in the 9th century. He was also the youngest person to bear the imperial title (aged 4 months), as well as the youngest to succeed as senior emperor (aged 2) in the Roman Empire. In 867, Michael was assassinated by his successor, Basil I.

Michael was the youngest child of the emperor Theophilos and his empress Theodora. His precise date of birth is uncertain, but the balance of available evidence supports a birthdate in early 840, probably on 9 or 10 January. He was crowned co-emperor soon after, probably on 16 May of the same year. Michael had just turned two years old when his father died, and Michael succeeded him as sole emperor on 20 January 842.

During Michael's minority, the empire was governed by a regency headed by his mother Theodora, her uncle Sergios, and the eunuch Theoktistos, who was Postal Logothete and the most powerful of the three. Within a year the regents had begun to carefully plan a program of restoring iconophile orthodoxy. Stories of Theodora's secret devotion to images during Theophilos' reign are dubious and she was likely motivated not only by religious but also political and pragmatic concerns. Theoktistos had been an enthusiastic supporter of Theophilos and so was clearly motivated by pragmatism in endorsing the later iconophile program. Yet there was little opposition other than from John VII and the clergy of the Hagia Sophia. The future Patriarch Methodios I held a synod in the Palace of Blachernae rather than in the patriarchal church, which suggests that they resisted the changes. However in March, the Acts of the Second Council of Nicaea (787) were reaffirmed and the leading iconoclasts of the Council of Constantinople of 815 declared to be heretics. Theophilos was not mentioned in order to not alienate the ruling imperial family and its supporters. Theodora had also explicitly demanded that her husband not be dishonoured. Patriarch John VII of Constantinople was then asked to resign, and replaced him with the iconophile Methodios, who, despite his iconophilism, was a close associate of Theophilos. The occasion was marked with a solemn procession on the first Sunday of Lent (11 March 843), from Blachernae to the Hagia Sophia, followed by a liturgy in the church. These events came to be known as the "Triumph of Orthodoxy". Methodios carried out an expulsion of iconoclast clergy and was later accused of fomenting dissension within the church. The sources, which are sympathetic to iconophilism, do not report much dissent among the clergy or general population, which suggests a general passivity or neutrality on the issue. With these events the second spell of Byzantine iconoclasm was put to an end, and the autonomy of the Church was affirmed against imperial power.

As the emperor was growing up, the courtiers around him fought for influence. Theoktistos disliked Michael's uncle Bardas, and excluded him from court politics. When Theodora and Theoktistos arranged the marriage of Eudokia Dekapolitissa to Michael, who preferred Eudokia Ingerina, Bardas won his nephew's favour by persuading him to allow a plot to murder Theoktistos. In 855, the regency was overthrown when Theoktistos was murdered in the Great Palace of Constantinople, and in 857, Theodora was barred from government and relegated along with her daughters to a monastery in 857.

The internal stabilisation of the state was not entirely matched along the frontiers. Although the Abbasid Caliphate was no longer launching major invasions, and raids were led by local amirs in Anatolia rather than the Caliph himself, Byzantine forces were defeated by the Abbasids in Pamphylia, Crete, and on the border with Syria, but a Byzantine fleet of 85 ships did score a victory over the Arabs in 853. There were also many operations around the Aegean and off the Syrian coast by at least three more fleets, numbering 300 ships total. The Byzantines were able to successfully assault Damietta in Egypt in 853. In the 820s, Andalusian Arab pirates seized Crete, which Theoktistos attempted to regain in the first year of the regency, though unsuccessfully. Bardas was planning to sail there but was murdered on the instigation of Basil in 866. The recapture was not achieved until 961, long after Michael's reign.

A conflict between the Byzantines and the First Bulgarian Empire occurred during 855 and 856. The Byzantine Empire wanted to regain its control over some areas of Thrace, including Philippopolis and the ports around the Gulf of Burgas on the Black Sea. Byzantine forces, led by the emperor and the caesar Bardas, were successful in reconquering a number of cities – Philippopolis, Develtus, Anchialus and Mesembria among them – as well as the region of Zagora. At the time of this campaign the Bulgarians were distracted by a war with the Franks under Louis the German and the Croatians. In 853, Khan Boris I of Bulgaria had allied himself to Duke Rastislav of Moravia against the Franks. The Bulgarians were heavily defeated by the Franks; following this, the Moravians changed sides and the Bulgarians then faced threats from Moravia.

Following an expedition led by Michael's uncle and general, Petronas, against the Paulicians from the eastern frontier and the Arab borderlands in 856, the imperial government resettled them in Thrace, thus cutting them off from their coreligionists and populating another border region. The Paulicians, whose power centre was Tephrike, were heavily persecuted after the restoration of icons as they were deemed unorthodox. It was only in 872 that Christopher, Domestic of the Schools, defeated their leader Chrysocheir, and Tephrike was taken in 878. Michael was also responsible, as per the writings of Constantine VII, for the subjugation of the Slavs settled in the Peloponnese.

Michael took an active part in the wars against the Abbasids and their vassals on the eastern frontier from 856 to 863, and particularly in 857 when he sent an army of 50,000 men against Emir Umar al-Aqta of Melitene. In 859, he personally led a siege on Samosata, but in 860 had to abandon the expedition to repel a naval attack by the Rus' on Constantinople. The Rus' fleet plundered the outer suburbs of the city but left of their own volition, probably because they had acquired sufficient booty. The subsequent relationship with the Rus' was mixed; they requested a Byzantine mission which was short-lived, and engaged in further raids but also trade in the tenth century. It was only in the late tenth century that the Christianisation of the Rus' was achieved by Grand Prince Vladimir of Kiev.