Ballast Key is an island in the Florida Keys in Monroe County, Florida, United States. It is the southernmost point of land in the contiguous United States. It was the last privately owned land within the boundaries of the Key West National Wildlife Refuge.
Located in the Outlying Islands of the Florida Keys, it is one of the Mule Keys that are west of Key West. The island's were donated by Key West developer David W. Wolkowsky to The Nature Conservancy, who deeded ownership of the land to the United States Fish and Wildlife Service while arranging to manage the resource.