This kingdom no longer exists, and its former borders are uncertain, but it has been situated in northern England or southern Scotland, in the Cumbrian region of north-west Britain, which was ruled by Urien and his son Owain during the late fourth and early fifth centuries AD, the traditional Arthurian period.

T. Clare, County Archaeologist of Cumbria, has been suggested that Urien was the historical original of Arthur, which would therefore mean that Rheged was Arthur's realm. This association seems to have sprung from Arthur's connections with Carlisle, which lies within the kingdom.

Sir Walter Scott, in The Bridal of Triermain, says that Arthur promised it to whichever of his knights married Gyneth, his daughter.