Northumberland, historic county and unitary authority of northeastern England. It is England’s northernmost county, bounded to the north by Scotland, to the east by the North Sea, to the west by the administrative county of Cumbria (historic county of Cumberland), and to the south by the county of Durham. Newcastle was the historic county town (seat) of Northumberland, but Morpeth is the unitary authority’s present administrative centre.Northumberland is an area of strongly contrasting landscapes, from the agricultural eastern coastal plain and the sparsely populated.