St. Lawrence Islands National Park is located in the Thousand Islands Region of the Saint Lawrence River. The islands are actually the worn-down tops of ancient mountains. This region, the Frontenac Axis, connects the Canadian Shield to the Adirondack Mountains in New York.

The park consists of 21 islands plus many smaller islets and a visitor centre at Mallorytown, Ontario on the mainland. This is Canada's smallest national park with a total area of 9 km².

Much of the park is only accessible by boat. There are picnic facilities and limited camping facilities on several islands.