The Illinois River is a tributary of the Michigan River, approximately 20 mi (32 km) in Jackson County in north central Colorado. It drains part of the North Park basin south of Walden.

It rises in the high Rabbit Ears Range of the Rocky Mountains along the continental divide at approximately 11500 ft, just south of Farview Mountain. It descends northward through a winding gorge, emerging into North Park at approximately 8,000,000 ft. It flows northward through the valley as a winding stream, past Rand. It passes under State Highway 14 just southeast of Walden and joins the Michigan from the north just north of Walden. A portion of the valley of the river south of Walden is located within the Arapaho National Wildlife Refuge.