Theophilus (813 - 842) was Byzantine emperor from 829 to 842. He was the second of the "Phrygian" dynasty.

Unlike his father Michael II, he declared himself a pronounced iconoclast. In 832 he issued an edict strictly forbidding the use of icons; but the stories of his cruel treatment of recalcitrants are so extreme that some think they are exaggerated.

At the time of his accession, the Sicilians were still engaged in hostilities with the Saracens, but Theophilus was obliged to devote all his energies to the war against the caliphs of Baghdad. This war was caused by Theophilus, who afforded an asylum to a number of Persian refugees, one of whom, called Theophobus after his conversion to Christianity, married the emperor's sister Helena, and became one of his generals.

The Byzantine army was at first successful; in 837 Samosata and Zapetra (Zibatra, Sozopetra), the birthplace of Caliph al-Mu'tasim, were taken and destroyed. Eager for revenge, al-Mu'tasim assembled a vast army, one division of which defeated Theophilus, who commanded in person, at Dasymon , while the other advanced against Amorium , the cradle of the Phrygian dynasty. After a brave resistance of fifty-five days, the city fell into al-Mu'tasim's hands through treachery on September 23, 838. Thirty thousand of the inhabitants were slain, the rest sold as slaves, and the city razed to the ground.

Theophilus never recovered from the blow; his health gradually failed, and he died at the beginning of 842. His character has been the subject of considerable discussion, some regarding him as one of the ablest of the Byzantine emperors, others as an ordinary oriental despot, an overrated and insignificant ruler. There is no doubt that he did his best to check corruption and oppression on the part of his officials, and administered justice with strict impartiality, although his punishments did not always fit the crime.

In spite of the drain of the war in Asia and the large sums spent by Theophilus on building, commerce, industry, and the finances of the empire were in a most flourishing condition, the credit of which was in great measure due to the highly efficient administration of the department. Theophilus, who had received an excellent education from John Hylilas , the grammarian, was a great admirer of music and a lover of art, although his taste was not of the highest. He strengthened the walls of Constantinople, and built a hospital, which continued in existence till the latest times of the Byzantine Empire.