This article argues that the rare Welsh territorial name ‘Rheinwg’ originally referred to the greater Dyfed of the eighth century, which stretched from Brycheiniog in east central Wales to the later heartlands of Dyfed in the south west. The name may have arisen in memory of Rhain ap Cadwgon, who by the ninth century would have been perceived as the common ancestor of two competing branches of the ruling dynasty.