It was made up from the near entirety of the ancient Pays de Saint-Brieuc, most of historical Trégor, the eastern half of Cornouaille, and the north-western part of the former diocese of Saint-Malo.

[4] On 27 February 1990, the name was changed to Côtes-d'Armor; the French word côtes means "coasts" and ar mor is "the sea" in Breton.

Côtes-d'Armor is part of the current administrative region of Brittany and is bounded by the departments of Ille-et-Vilaine to the east, Morbihan to the south, and Finistère to the west, and by the English Channel to the north.

Towards its western extremity this chain bifurcates to form the Montagnes Noires in the south-west and the Monts d'Arrée in the west of the department.

Côtes-d'Armor's long tradition of anti-clericalism, especially in the interior around Guingamp (a former Communist stronghold), has often led to the department's being seen as an area of left-wing exceptionalism in a region that historically was otherwise strongly Catholic and right-wing.