The Biddeford-Saco Mills Historic District encompasses the historic mill complex that flanks both sides of the Saco River in Biddeford and Saco, Maine. It covers 38 acres (15 ha) of property adjacent to both cities' downtown areas which has seen industrial uses since the 18th century, and presently contains a collection of well-preserved 19th and early-20th century industrial buildings. The district was added to the National Register of Historic Places in 2008. Many of the buildings in the district have been rehabilitated and repurposed for other uses, including residential, commercial, and industrial.