The Cotentin peninsula is part of the Armorican Massif (with the exception of the Plain lying in the Paris Basin) and lies between the estuary of the Vire river and Mont Saint-Michel Bay. It is divided into three areas: the headland of Cap de la Hague, the Cotentin Pass (the Plain), and the valley of the Saire River (Val de Saire). It forms the bulk of the department of Manche. Its southern part, known as "le Marais" (the Marshlands), crosses from east to west from just north west of Saint Lo and east of Lessay and marks a natural border with the rest of Manche.
The largest town in the peninsula is Cherbourg on the north coast, a major cross-channel port.
The western coast of the peninsula, known as the Côte des Îles ("Islands Coast"), faces the Channel Islands. Ferry links serve Carteret and the islands of Jersey, Guernsey and Alderney from Dielette. Off the east coast of the peninsula lies the island of Tatihou and the Îles Saint-Marcouf.
The oldest stone in France is found in outcroppings on the coast of Cap de la Hague, at the tip of the peninsula.
Cotentin was almost an island at one time. Only a small strip of land in the heath of Lessay connected the peninsula with the mainland. Thanks to the so-called portes à flot (fr), which close at flood and open at ebb and which were built in the west coast and in the Baie des Veys, on the east coast, the Cotentin has become a peninsula.
The Côte des Havres lies between the Cape of Carteret and the Cape of Granville. To the northwest, there are two sand dune systems: one stretching between Siouville-Hague and Vauville, the other one stretching between Cap of Carteret and Baubigny.