Keizer is a city located in Marion County, Oregon, United States, along the 45th parallel. As of the 2020 United States Census, its population was 39,376, making it the 14th most populous city in Oregon. It lies in the Willamette Valley, and is part of the Salem Metropolitan Statistical Area. It was named after pioneer Thomas Dove Keizur and his family, who arrived in the Wagon Train of 1843, and later filed donation land claims.

Marion County is one of the 36 counties in the U.S. state of Oregon. The population was 345,920 at the 2020 census, making it the 5th most populous county in Oregon. The county seat is Salem, which is also the state capital of Oregon. The county was originally named the Champooick District, after Champoeg, a meeting place on the Willamette River. On September 3, 1849, the territorial legislature renamed it in honor of Francis Marion, a Continental Army general from South Carolina who served in the American Revolutionary War.