The Kingdom of Northumbria is a medieval kingdom of the Angles, in what is now Northern England and South-East Scotland, becoming subsequently an earldom in a united Anglo-Saxon Kingdom of England.

The name reflects the approximate southern limit to the kingdom's territory, the Humber estuary.Northumbria is formed by Æthelfrith in central Great Britain in Anglo-Saxon times.

At the beginning of the 7th century, the two kingdoms of Bernicia and Deira are unified.

(In the 12th century writings of Henry of Huntingdon the kingdom will be defined as one of the Heptarchy of Anglo-Saxon kingdoms.)

At its greatest, the kingdom extends at least from just south of the Humber, to the River Mersey and to the Forth (roughly, Sheffield to Runcorn to Edinburgh) — and there is some evidence that it may have been much greater.The later (and smaller) earldom comes about when the southern part of Northumbria (ex-Deira) is lost to the Danelaw.

The northern part (ex-Bernicia) at first retains its status as a kingdom but when it becomes subordinate to the Danish kingdom it has its powers curtailed to that of an earldom, and retains that status when England is reunited by the Wessex-led reconquest of the Danelaw.

The earldom os bounded by the River Tees in the south and the River Tweed in the north (broadly similar to the modern North East England).