Sanilac County is a county located in the Thumb region of the U.S. state of Michigan. As of the 2020 Census, the population was 40,611. The county seat is Sandusky. The county was created on September 10, 1822, and was fully organized on December 31, 1849. Sanilac County is considered to be part of the Thumb of Michigan, a subregion of the Flint/Tri-Cities. Sanilac County enjoys seasonal tourism in towns such as Lexington, Port Sanilac, and Carsonville. Sanilac County is economically attached to St. Clair County and Huron County and is largely composed of nearly flat areas of rich soil.

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.