Îles Saint-Marcouf is a group of two small uninhabited islands off the coast of Normandy, France. They lie in the Baie de la Seine region of the English Channel and are 6.5 km (4.0 mi) east of the coast of the Cotentin peninsula at Ravenoville and 13 km (8.1 mi) from the island of Tatihou and the harbour at Saint-Vaast-la-Hougue.
In addition to the fortifications on the larger island there is a lighthouse, which dates to 1948.The larger island, île du Large, is 500 metres (1,600 ft) east of the smaller île de Terre. They have a total area of 1,400 hectares (3,500 acres) and a maximum altitude of 10 meters (33 ft).The islands take their name from Saint Marcouf, a saint born in Bayeux, whom it was said could cure anyone of scrofula. he died on the Îles Saint-Marcouf on 1 May 588. There was a monastic presence on the islands until the 15th century. 

The French government directly administers the islands, which have the status of a protected nature reserve with restricted access. Île de Terre has been a designated nature preserve since 1967. Île du Large has been off-limits since 1991 for reasons of safety. The primary bird species are seagulls and cormorants. In winter tens of thousands of seagulls shelter on the islands. Access to the islands is strictly forbidden.

Currently, the fortifications are falling into ruins. Since 2003, the association "les Amis de l'île du Large Saint-Marcouf" (Friends of Saint-Marcouf), together with students from the Collège de Carentan, have initiated a campaign to convince the authorities once again permit recreational access to the Île du Large. In 2009 the Friends of Saint-Marcouf received permission to commence preservation work on the fortifications.

In World War II the islands became the first French territory that seaborne Allied forces took on D-Day. At 04:30 on 6 June 1944 four US soldiers, armed only with knives, swam ashore from two-man canoes. When they had verified that the islands were unoccupied, 132 troops from 4th and 24th Squadrons of the U.S. 4th Cavalry Group landed on the islands to secure the approaches to Utah Beach. Although they faced no resistance, the US troops suffered 19 casualties, killed and wounded, from mines that the Germans had left.