Keizer is a city located in Marion County, Oregon, United States, along the 45th parallel. As of the 2010 United States Census, its population was 36,478. It lies in the Willamette Valley and is part of the Salem Metropolitan Statistical Area. It was named after brothers Thomas Dove (T. D.) and John Brooks (J. B.) Keizur, two pioneers who arrived in the Wagon Train of 1843, and later filed donation land claims.