Marion County is a county located in the Willamette Valley region of the U.S. state of Oregon. The population was 315,335 at the 2010 census. The county seat is Salem. It 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 of the American Revolutionary War.
Marion County is part of the Salem Metropolitan Statistical Area.
Marion County was created on July 5, 1843, one of the original four districts of the Oregon territory 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.