Placentia, CA. Online Offers. Placentia is a city in northern Orange County, California. The population was 50,533 at the 2010 census, up from 46,488 at the 2000 census. This includes the community of Atwood, which is included in the city of Placentia, and is located in its southernmost quadrant. Primarily known as a bedroom community, Placentia is known for its quiet neighborhoods and strong public safety. In the southwest, or downtown area, is the Placita-Santa Fe District, which boasts some of the best Mexican restaurants in Orange County.