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.

Salem is the capital city of the U.S. state of Oregon, and the county seat of Marion County. It is located in the center of the Willamette Valley alongside the Willamette River, which runs north through the city. The river forms the boundary between Marion and Polk counties, and the city neighborhood of West Salem is in Polk County. Salem was founded in 1842, became the capital of the Oregon Territory in 1851, and was incorporated in 1857.