Placer County, officially the County of Placer, is a county located in the U.S. state of California. As of the 2020 census, the population was 404,739. The county seat is Auburn. Placer County is included in the Greater Sacramento metropolitan area. It is in both the Sacramento Valley and Sierra Nevada regions, in what is known as the Gold Country. The county stretches roughly 65 miles from Sacramento's suburbs at Roseville to the Nevada border and the shore of Lake Tahoe.