Pen Mar is an unincorporated community in Washington County, Maryland, United States. It is one of the border towns in the United States with a portmanteau name. Located on the Mason–Dixon line, it is bordered to the north by the community of Pen Mar, Pennsylvania. The community came to prominence as a resort in the 20th century after the construction of Pen Mar Park by the Western Maryland Railway. While the resort has long been dismantled, the area still has some significance in Maryland, since it is the northern starting point for the Maryland section of the Appalachian Trail.