Offa, the ruler of the Anglo-Saxon Kingdom of Mercia in the late eighth century, is best remembered for the immense barrier that has long had his name: Offedik, in an earlier regional form, and Clawdd Offa in Welsh. Although the history of the construction of the wall remains obscure, the monk Asser notes (in his biography of King Alfred a century later) that Offa commanded a barrier to be built from the north to the south coast of Wales. Not all of the wall has survived, but some parts are in good condition as in the segment shown at left.

While the defense of Mercia from the Welsh seems to have been its main purpose, Offa's Dyke has continued to mark (approximately) the border between Wales and England up to the present, and nowadays many Welsh as well as English people take pride in the monument.