Mecosta County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 42,798. The county seat is Big Rapids.

The county is named after Chief Mecosta, the leader of the Potawatomi Native American tribe who once traveled the local waterways in search of fish and game. Chief Mecosta was one of the signers the Treaty of Washington in 1836. The easily navigated waterways soon led to a boom in lumber industry growth. Workers settled the area in 1851, and the county was officially settled and the government officially organized in 1859. Mecosta County is home to over 100 lakes, rivers, and streams with the Muskegon River winding its way through the county seat and largest city Big Rapids (originally named Leonard).