Allegan County is a county in the U.S. state of Michigan. As of the 2020 United States Census, the population was 120,502. The county seat is Allegan. The name was coined by Henry Rowe Schoolcraft to sound like a Native American word. Allegan County comprises the Holland, Michigan Micropolitan Statistical Area, which is included in the Grand Rapids–Kentwood–Muskegon Combined Statistical Area.