Flat Rock is a city in Wayne County, Michigan, United States, with a small portion of the city extending into Monroe County. At the 2020 census, the population was 10,541.

Wayne County is the most populous county in the U.S. state of Michigan. As of 2020, the United States Census placed its population at 1,793,561, making it the 19th-most populous county in the United States. The county seat is Detroit. The county was founded in 1796 and organized in 1815. Wayne County is included in the Detroit-Warren-Dearborn, MI Metropolitan Statistical Area. It is one of several U.S. counties named after Revolutionary War-era general Anthony Wayne.