Adeline Jay Geo-Karis Illinois Beach State Park, part of the Illinois state park system, is located along Lake Michigan in northern Lake County in northeast Illinois. Together with lands to the north, including Chiwaukee Prairie, it forms the Chiwaukee Prairie Illinois Beach Lake Plain, an internationally recognized wet-land of importance under the Ramsar Convention. The park is broken into two units that encompass an area of 4,160 acres (1,683 ha) and contains over six miles of Lake Michigan shoreline. In 2010, it was renamed for former state senator Adeline Geo-Karis.

Lake Michigan is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume and the third-largest by surface area, after Lake Superior and Lake Huron. To the east, its basin is conjoined with that of Lake Huron through the 3+1⁄2-mile (5.6-kilometer) wide, 295-foot deep Straits of Mackinac, giving it the same surface elevation as its easterly counterpart; geologically, the two bodies are a single lake that is, by area, the largest freshwater lake in the world.