Mercia, sometimes spelled Mierce, was one of the kingdoms of the Anglo-Saxon heptarchy, in what is now England, in the region of the Midlands, with its heart in the valley of the River Trent and its tributary streams. Mercia's neighbours included Northumbria, Powys, the kingdoms of southern Wales, Wessex, Sussex, Essex, and East Anglia. The term survives today in the name of the West Mercia Constabulary and also in the new British Army regiment, the Royal Mercian Regiment.
Mercia's exact evolution from the Anglo-Saxon invasions is more obscure than that of Northumbria, Kent, or even Wessex. Archeological surveys show that Angles settled the lands north of the River Thames by the sixth century. The name Mercia is Old English for "boundary folk" (see marches), and the traditional interpretation was that the kingdom originated along the frontier between the Welsh and the Anglo-Saxon invaders, although P. Hunter Blair has argued an alternative interpretation that they emerged along the frontier between the kingdom of Northumbria and the inhabitants of the Trent river valley.