Chain O'Lakes State Park is a two thousand seven hundred ninety-three-acre Illinois state park where the Fox River enters the Chain O'Lakes in Lake and McHenry counties north of Chicago. Within a corridor of suburban open space and waterways, it preserves part of one of the region's best-known lake and river landscapes. Its importance comes from its location at the inlet to the Chain O'Lakes, where connected water, marsh, and river habitat create a broader ecological setting than a single lake. Recognized as a centerpiece of the proposed Hackmatack National Wildlife Refuge, it combines river access, protected wildlife habitat, and conservation value on the edge of a major metropolitan area.