The Sack of Amorium by the Abbasid Caliphate in mid-August 838 is one of the major events in the long history of the Arab–Byzantine Wars.

The Abbasid campaign is led personally by the Caliph al-Mu'tasim (reigned 833–842), in retaliation to a virtually unopposed expedition launched by the Byzantine emperor Theophilos (r. 829–842) into the Caliphate's borderlands the previous year.

Mu'tasim targets Amorium, a Byzantine city in western Asia Minor (modern Anatolia), because it is the birthplace of the ruling Byzantine dynasty and, at the time, one of Byzantium's largest and most important cities.

The caliph gathers an exceptionally large army, which he divides in two parts.

The Abbasids penetrate deep into Byzantine-held Asia Minor, while the northern army defeats the Byzantine forces under Theophilos at Anzen.

The Abbasid troops then converge upon Ancyra, which they find abandoned.

After sacking the city, they turn south to Amorium, where they arrive on 1 August.

Faced with intrigues at Constantinople and the rebellion of the large Khurramite contingent of his army, Theophilos is unable to aid the city.Amorium is strongly fortified and garrisoned, but a traitor reveals a weak spot in the wall, where the Abbasids concentrate their attack, effecting a breach.

Unable to break through the besieging army, the commander of the breached section privately attempt to negotiate with the Caliph.

He leaves his post, which allows the Arabs to take advantage, enter the city and capture it.

Amorium is systematically destroyed, never to recover its former prosperity.

Many of its inhabitants are slaughtered, and the remainder driven off as slaves.

Most of the survivors are released after a truce in 841, but prominent officials are taken to the caliph's capital of Samarra and executed years later after refusing to convert to Islam, becoming known as the 42 Martyrs of Amorium.The conquest of Amorium is not only a major military disaster and a heavy personal blow for Theophilos, but also a traumatic event for the Byzantines, its impact resonating in later literature.

The sack does not ultimately alter the balance of power, which is slowly shifting in Byzantium's favor, but it thoroughly discredits the theological doctrine of Iconoclasm, ardently supported by Theophilos.

As Iconoclasm relies heavily on military success for its legitimization, the fall of Amorium contributes decisively to its abandonment shortly after Theophilos's death in 842.