The Great Lakes-St. Lawrence Lowlands, or simply St. Lawrence Lowlands, is a physiographic region of Eastern Canada that comprises a section of southern Ontario bounded on the north by the Canadian Shield and by three of the Great Lakes — Lake Huron, Lake Erie and Lake Ontario — and extends along the St. Lawrence River to the Strait of Belle Isle and the Atlantic Ocean. The lowlands comprise three sub-regions that were created by intrusions from adjacent physiographic regions — the West Lowland, Central Lowland and East Lowland. The West Lowland includes the Niagara Escarpment, extending from the Niagara River to the Bruce Peninsula and Manitoulin Island. The Central Lowland stretches between the Ottawa River and the St. Lawrence River. The East Lowland includes Anticosti Island, Îles de Mingan, and extends to the Strait of Belle Isle.

The St. Lawrence Lowlands is one of the most densely populated, prosperous and productive regions in Canada. Major urban areas include Toronto, Montreal, Ottawa-Gatineau and Quebec City.

The St. Lawrence Lowlands was covered by surficial deposits left by ice sheets following the Pleistocene glaciations. It is the smallest of Canada's seven physiographic regions — the others being the Arctic Lands, the Cordillera, the Interior Plains, the Canadian Shield, the Hudson Bay Lowlands and the Appalachian Uplands — distinguished by topography and geology. The boundaries of the area largely reflect that of the Mixedwood Plains Ecozone, the smallest of Canada's fifteen terrestrial ecozones.

The Great Lakes-St. Lawrence Lowlands includes a section of southern Ontario bounded on the north by the Canadian Shield and by three of the Great Lakes—Lake Huron, Lake Erie, and Lake Ontario, and extends along the Saint Lawrence River to the Strait of Belle Isle The St. Lawrence Lowlands ecoregion includes the Ottawa River and the St. Lawrence River lowlands. The Frontenac Axis, a wedge of the Canadian Shield that protruded southwards into the northern United States, separates the St. Lawrence Lowlands in Quebec from southern Ontario. In the north the Lowlands are bounded by the Laurentian Highlands and to the south by the Eastern Quebec Uplands. The Geological Survey of Canada published an updated map in 2014 defining the boundaries of the Great Lakes-St. Lawrence Lowlands physiographic region.

The Great Lakes-St. Lawrence's lowlands include three sub-regions.

The southwest region of the Lowlands which is in southern Ontario and northern New York and Vermont is divided by the Niagara Escarpment, which extends northwest from the Niagara River to the Bruce Peninsula and Manitoulin Island. Most of the area which is east of the Niagara Escarpment—from Lake Ontario north to Georgian Bay—has low relief. The characteristic of this area, which was "entirely covered by glaciers during parts of the Pleistocene", are "lakes, poorly drained depressions, morainic hills, drumlins, eskers, outwash plains, and other glacial features." Soils in this area include "peat, muck, marl, clay, silt, sand, and gravel" This subregion includes the Bruce Peninsula to the northwest and the Niagara Peninsula, which is the warmest and most intensely cultivated part of the ecozone.

The Central Lowland stretches between the Ottawa River and the Saint Lawrence River and extends to Québec city. Major cities include Trois-Rivières. The Lowlands include a small area on the north shore near Québec City. The St. Lawrence Lowlands are underlain by marine and lacustrine clays and bedrock outcrops of Paleozoic limestone. In the north it is bounded by the Laurentian Highlands.

The East Lowland includes Anticosti Island, Îles de Mingan, the Strait of Belle Isle and the Newfoundland Coastal Lowland. To the south it is bordered by the Eastern Quebec Uplands.