Ahtum (also spelled Ohtum or Achtum) was an early 11th century duke of Banat (now divided between Romania and Serbia) and a descendant of Glad, another local duke.

We also know about him that he was of Christian Orthodox faith, an ally of the Byzantine Empire and was sustained by an army of Cumans, Bulgarians and Vlachs (Romanians). He was eventually defeated by the Hungarian Kingdom. The date of the defeat is not clear, some historians suggesting 1028, while others around 1002. After his death, the Banat is included in the Hungarian Kingdom.

The main historical source about him was the legend of Saint Gerard and he is also mentioned in the Anonymus' Gesta Hungarorum. The mentioning in the Legend because it has nothing to do with the life of Saint Gerard, his story being probably inserted in order to emphasise a victory of a Roman Catholic-rite ruler against an Orthodox rite, at a time when the Papality was trying to expand their influence in South-Eastern Europe.