Plainwell is a city in Allegan County in the U.S. state of Michigan. The population was 3,933 at the 2000 census. Plainwell is located on M-89 just east of its junction with U.S. 131. The city of Otsego is about three miles (5 km) to the west. Kalamazoo is about ten miles (16 km) to the south and Grand Rapids about 35 miles (56 km) north. The Kalamazoo River flows through the center of the city.