Diuma (or Dwyna or Duma) was the first Bishop of Mercia in the Anglo-Saxon Kingdom of Mercia, during the Early Middle Ages.
Diuma was an Irishman, and was one of four priests from the Kingdom of Northumbria, who went to Mercia in 653 to introduce the Christian faith to the region. Their coming had been requested by the Mercian ealdorman, Peada, son of Penda, King of Mercia. Peada had become a Christian when he married Alhflaed, daughter of Oswiu, King of Northumbria. They arrived under the leadership of Cedd.
After Penda's death, Diuma was consecrated a bishop by Finan. He established his see in Repton, but the exact boundaries of the bishopric are unclear. The Venerable Bede claimed that he was bishop of both the Middle Angles and the Mercians.
Diuma was consecrated after 655 but his death date is unknown. It would appear to have been not long after this, as he was succeeded as bishop by Ceollach, whose own successor, Trumhere, was named bishop around 658. Bede refers to his episcopate as having been fruitful but short, after which his body was buried in Charlbury, in the territory of the Middle Angles, where the foundation of the parish church is attributed to him.
In 669 the seat of the diocese was moved by a successor, Chad, to Lichfield.