Ontario, two times as large as France, covers some 412,579 square miles (1,068,580 square kilometers) and is bordered on the north by Hudson Bay; on the east by Québec; on the south by the St. Lawrence River, the Great Lakes, and the US state of Minnesota; and on the west by Manitoba.

Three main geological regions make up Ontario: the Great Lakes-St. Lawrence Lowlands, the Canadian Shield, and the Hudson Bay Lowlands. The Hudson Bay Lowlands are narrow coastal plains bordering Hudson Bay and James Bay; the land is wet and covered by scrub growth. The Canadian Shield, covering the rest of northern Ontario from Lake Superior to Hudson Bay, and extending into the southern part of the province, is a vast rocky plateau. Although the soil is poor and not well suited to large-scale farming, there is a wealth of minerals, forests, and water power. The Canadian Shield and the Hudson Bay Lowlands cover 90 percent of the province's territory. Four of the five Great Lakes are the most visible results of the ice age in Ontario, providing the longest fresh water beach in the world. The biggest, Lake Superior, is the world's largest body of fresh water. About 68,490 square miles (177,390 square kilometers), or one-sixth of Ontario's terrain, is covered by some 400,000 lakes and 37,000 miles (59,000 kilometers) of rivers.

The Great Lakes–St. Lawrence Lowlands comprise the rest of southern Ontario. Here is where most of Ontario's population can be found; it is also the area with the most of province's industry, commerce, and agricultural lands.

The short Niagara River, which flows from Lake Erie into Lake Ontario is the site of Niagara Falls (at the Ontario–New York border), which drains some 800,000 gallons (3,000,000 liters) of water per second over its 187-foot (57-meter) drop. The highest point in Ontario is found at Ishpatina Ridge in the Timiskaming District, at an elevation of 2,274 feet (693 meters).