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 of the American Revolutionary War.
Marion County was created by the Provisional Legislature of Oregon on July 5, 1843 as the Champooick District, one of the original four districts of the Oregon Country along with Twality (later Washington), Clackamas, and Yamhill counties. The four districts were redesignated as counties in 1845.
Originally, this political entity stretched southward to the California border and eastward to the Rocky Mountains. With the creation of Wasco, Linn, Polk, and other counties, its area was reduced in size. Marion County's present geographical boundaries were established in 1856.
In 1849, Salem was designated the county seat. The territorial capital was moved from Oregon City to Salem in 1852. The ensuing controversy over the location of the capital was settled in 1864 when Salem was confirmed as the state capital.