Marion County is a county located in the U.S. state of Oregon. The population was 315,335 at the 2010 census, making it the fifth-most populous county in Oregon. The county seat is Salem, the state capital. The county was originally named the Champooick District, after Champoeg (earlier Champooick), 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.