Saco is a city in York County, Maine, United States. The population was 18,482 at the 2010 census. It is home to Ferry Beach State Park, Funtown Splashtown USA, Thornton Academy, as well as General Dynamics Armament Systems, a subsidiary of the defense contractor General Dynamics. Saco sees much tourism during summer months, due to its amusement parks, Ferry Beach State Park, and proximity to Old Orchard Beach.Saco is part of the Portland?South Portland?Biddeford, Maine metropolitan statistical area. Saco's twin-city is Biddeford.