St. Catharines is the most populous city in Canada's Niagara Region, the eighth largest urban area in the province of Ontario. As of 2017, St. Catharines has an area of 96.13 square kilometres (37.12 sq mi) and 140,370 residents. It lies in Southern Ontario, 51 kilometres (32 mi) south of Toronto across Lake Ontario, and is 19 kilometres (12 mi) inland from the international boundary with the United States along the Niagara River. It is the northern entrance of the Welland Canal. Residents of St. Catharines are known as St. Catharinites. St. Catharines carries the official nickname "The Garden City" due to its 1,000 acres (4 km2) of parks, gardens, and trails.

The Regional Municipality of Niagara, also colloquially known as the Niagara Region or Region of Niagara, is a regional municipality comprising twelve municipalities of Southern Ontario, Canada. The regional seat is in Thorold. It is the southern end of the Golden Horseshoe, the largest megalopolis in Canada.