Washington Boro is an unincorporated community and census-designated place (CDP) in Manor Township, Lancaster County, Pennsylvania, United States, along the Susquehanna River. The ZIP code is 17582. It is served by the Penn Manor School District and is the terminus of Pennsylvania Route 441 and Pennsylvania Route 999. As of the 2010 census, the population was 729.

The Susquehanna River is a major river located in the Mid-Atlantic region of the United States, overlapping between the lower Northeast. At 444 miles (715 km) long, it is the longest river on the East Coast of the United States. By watershed area, it is the 16th-largest river in the United States, and also the longest river in the early 21st-century continental United States without commercial boat traffic.