Theophilos (Greek: Θεόφιλος, romanized: Theóphilos, sometimes Latinised as Theophilus; c. 812 – 20 January 842) was Byzantine Emperor from 829 until his death in 842. He was the second emperor of the Amorian dynasty and the last emperor to support iconoclasm.

Theophilos was well-educated in the imperial household. Upon his accession to the throne he faced the dual threat of the Abbasid Caliphate in Asia Minor and the Aghlabids in Sicily, and personally led armies against both foes from 831 onwards. He won fleeting victories but the retaliation of Caliph al-Mu'tasim (r. 833–842) was devastating, as was most humiliating in the Sack of Amorion in 838, the ancestral home of Theophilos' Amorian dynasty. Internal strife within the Caliphate allowed the Byzantines to recover. Theophilos engaged in many construction and renovation projects. One of his closest allies was the learned and cosmopolitan John the Grammarian, and they both improved relations with the Caliph and appreciated Arabic culture. He also secured nominal overlordship over the Principality of Serbia, uniting against their common enemy, the First Bulgarian Empire.

The military defeats to the Arabs inspired a more intensely iconoclastic policy than what Theophilos inherited from his father Michael II, and he persecuted many clerics for refusing to submit to the imperial will. After his death, his wife Theodora rescinded this policy but also defended his broader reputation. Accordingly the historical record preserves a picture of Theophilos as a just ruler and keen constructor and administrator, alongside rebukes of his iconoclasm.

Theophilos was born in late 812 or early 813, the son of Emperor Michael II and his wife Thekla, and the godson of Emperor Leo V the Armenian. Michael II crowned Theophilos co-emperor in 821. The date is almost universally given as 12 May 821 (Whitsunday), although this is not really corroborated by any source (another possible date is 24 March, Easter). Unlike his father, Theophilos received an extensive education from John Hylilas, the grammarian, and was a great admirer of music and art.

On 2 October 829, Theophilos succeeded his father as sole emperor. Theophilos continued in his predecessors' iconoclasm, though without his father's more conciliatory tone, issuing an edict in 832 forbidding the veneration of icons. He also saw himself as the champion of justice, which he served most ostentatiously by executing his father's co-conspirators against Leo V immediately after his accession.

At the time of his accession, Theophilos was obliged to wage wars against the Arabs on two fronts. Sicily was once again invaded by the Arabs, who took Palermo after a year-long siege in 831, established the Emirate of Sicily, and gradually continued to expand across the island. In 830, Abbasid Caliph al-Ma'mun (r. 813–833) invaded Cappadocia in response to Theophilos' attack on Mopsuestia and Tarsos, capturing the city of Tyana. He launched a second campaign in 831 and then a third in 832, in which he captured the major border fortress of Loulon, but in 833 failed to seize Amorion. Another defeat in Cappadocia forced Theophilos to sue for peace (a one-off tribute of 100,000 gold dinars and the return of 7,000 prisoners), which he secured the next year, after the death of al-Ma'mun in 833. He was succeeded by his brother al-Mu'tasim (r. 833–842) who abandoned Tyana.

During the respite from the war against the Abbasids, Theophilos arranged for the abduction of the Byzantine captives settled north of the Danube by Krum of Bulgaria. The rescue operation was carried out with success in c. 836, and the peace between Bulgaria and the Byzantine Empire was quickly restored. However, it proved impossible to maintain peace in the East. In 834 Theophilos had given asylum to a number of refugees from the east; one of the group was Nasr, a Persian who was baptized, changed his name to Theophobos, married the Emperor's aunt Irene and became one of his generals. As relations with the Abbasids deteriorated, Theophilos prepared for a new war.

In 837, Theophilos led a vast army of 70,000 men towards Mesopotamia and captured Melitene, Arsamosata and Samosata. The Emperor also took and destroyed Sozopetra, which some sources claim as the birthplace of Caliph al-Mu'tasim. Theophilos returned to Constantinople in triumph. Eager for revenge, al-Mu'tasim assembled a vast army and launched a three-pronged invasion of Anatolia in 838. Theophilos decided to strike one division of the caliph's army before they could combine. On 21 July 838, at the Battle of Anzen in Dazimon, Theophilos personally led a Byzantine army of 25,000 to 40,000 men against the troops commanded by al-Afshin. Afshin withstood the Byzantine attack, counter-attacked, and won the battle. The Byzantine survivors fell back in disorder and did not interfere in the Caliph's continuing campaign.