Plainwell is a city in Allegan County in the U.S. state of Michigan. The population was 3,804 at the 2010 census. Plainwell is located on M-89 just east of its junction with US 131. The city of Otsego is about three miles (4.8 km) to the west. The city of Kalamazoo is about ten miles (16 km) to the south, and the city of Grand Rapids is about thirty five miles (56 km) to the north. The Kalamazoo River flows through the center of the city and it is known as "The Island City".