Deira (from Brythonic Deifr, meaning "waters") was a kingdom in England during the 6th century AD. It later merged with the kingdom of Bernicia (Brythonic, Brynaich) to the north to form the kingdom of Northumbria.

According to Simeon of Durham it extended from the Humber to the Tyne, but the land was waste north of the Tees. York was the capital of its kings.

The date of its first settlement is quite unknown, but the first king of whom we have any record is Ælla, who flourished in the later 6th century. After his death, Deira was subject to king Æthelfrith of Bernicia, who united the two kingdoms into Northumbria. Æthelfrith ruled until the accession of Ælla's son Edwin, in 616 or 617, who also ruled both kingdoms until 633.

Osric, the nephew of Edwin, ruled Deira after Edwin, but his son Oswine was put to death by Oswiu in 651. For a few years subsequently Deira was governed by Aethelwald son of Oswald.

Bede wrote of Deira in his Historia Ecclesiastica.

For a list of the kings of Deira, see: List of monarchs of Northumbria.