The Cass River is a 61.5-mile-long (99.0 km) river in the Thumb region of the U.S. state of Michigan. It drains large portions of Sanilac and Tuscola counties and smaller portions of Genesee, Huron, Lapeer, and Saginaw counties.

The Thumb is a region and a peninsula of the U.S. state of Michigan, so named because the Lower Peninsula is shaped like a mitten. The Thumb area is generally considered to be in the Central Michigan region, east of the Tri-Cities and north of Metro Detroit. The region is also branded as the Blue Water Area.