Eidyn was the region around modern Edinburgh in sub-Roman and early medieval Britain, approximately during the 5th–7th centuries. It centred on the stronghold of Din Eidyn, thought to have been at Castle Rock, now the site of Edinburgh Castle, and apparently included much of the area below the Firth of Forth. It was the most important district of the Brittonic kingdom of Gododdin, and a significant power in the Hen Ogledd, or Old North, the Brittonic-speaking area of what is now southern Scotland and northern England.
The site of Din Eidyn has been nearly continuously occupied since the Bronze Age, serving as a stronghold of the Votadini during the Roman era and later the principal centre of their successors, the Gododdin kingdom. Eidyn's importance to the Hen Ogledd is reflected in the medieval poem Y Gododdin, which concerns a war band that gathered there for a raid around AD 600. After years of decline, Eidyn was conquered by the Angles in 638.