The island is boomerang-shaped (one end pointing roughly west, the other roughly north) and is the largest island in the Hochelaga Archipelago. The St. Lawrence widens into Lac Saint-Louis south-west of the island, narrows into the Rapides de Lachine, then widens again into the Bassin de la Prairie before becoming the St. Lawrence again and flowing toward Québec. Île Sainte-Hélène and Île Notre-Dame are in the Saint Lawrence southeast of downtown Montreal.
The Ottawa widens and becomes Lac des Deux-Montagnes north-west of the island. The Sainte-Anne Canal, between the western tip of the island and Île-Perrot, connects Lac des Deux-Montagnes and Lac Saint-Louis. Another outlet of Lac des Deux-Montagnes, the Rivière des Prairies, flows along the north shore of the island and into the St. Lawrence at the northeastern tip of the island.
The island is approximately 50 km long and 16 km wide at its widest point. It has 266,6 km of coastline. At its centre are the three peaks of Mount Royal. The south-west of the island is separated by the Lachine Canal between Lachine and Montreal's Old Port.
The island of Montreal is the major component of the territory of the city of Montreal, along with Île-Bizard, Île-Dorval, Île Sainte-Hélène, Île Notre-Dame, Île des Soeurs, and some 69 smaller islands. The island formerly contained 26 municipalities, all of which were merged into the City of Montreal on January 1, 2002.
The island derived its name from Mount Royal, and gradually spread its name to the town, which had originally been called Ville-Marie. In the Iroquois language, the island is called Tiohtià:ke Tsi or Ka-wé-no-te.