Belle-Île (French pronunciation: [bɛl il]), Belle-Île-en-Mer (French pronunciation: [bɛl il ɑ̃ mɛʁ]), or Belle Isle (Breton: Ar Gerveur, [arˌɡɛrˈvøːr]; Old Breton: Guedel) is a French island off the coast of Brittany in the département of Morbihan, and the largest of Brittany's islands. It is 14 kilometres (7+1⁄2 nautical miles) from the Quiberon peninsula.

Administratively, the island is divided into four communes:

Belle-Île formed a canton until 2015 when it was merged into the canton of Quiberon as part of a general overhaul.

The island measures 17 by 9 kilometres (10+1⁄2 by 5+1⁄2 miles) and has an average elevation of 40 metres (130 feet). The area is about 84 km² (32+1⁄2 sq mi). The coasts are a mixture between dangerously sharp cliff edges on the southwest side, the Côte Sauvage ("wild coast"), and placid beaches, the largest being les Grands Sables ("the great sands") and navigable harbours on the northeast side. The island's climate is oceanic, having less rain and milder winters than on the mainland.

The two main ports are Le Palais (accessible by ferry from Quiberon, Port-Navalo and Vannes) and Sauzon (accessible by ferry from Quiberon and Lorient).

There used to be forests on the island, but these have long disappeared due to increasing agricultural use of the land.

Belle Île experiences a warm-summer Mediterranean climate.

At the 2009 census, the population of Belle Île was 4,920. However, in summer the population may increase to 25,000, with a peak at 35,000 between 15 July and 15 August due to tourist activity.