Keizer is a city 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.