Cook's Bay is the southernmost bay of Lake Simcoe in Ontario, Canada. Most of the southern part of the bay is within the jurisdiction of Georgina, a town in the Regional Municipality of York. Its western end is in Simcoe County, and a small portion from the mouth of the Holland River to the Simcoe County border is within Bradford West Gwillimbury. The largest community on the shores of the bay is Keswick.