San Juan County is located in the Salish Sea in the northwest corner of Washington State. It is bordered on the northeast by Whatcom County, the east by Skagit County, the southeast by Island County, the south by Jefferson County and the southwest by Clallam County. As of the 2010 U.S. Census, San Juan County had a total land area of 173.92 square miles and a population of 15,769. Friday Harbor is the largest city with a population of 2,180. Also of interest:
- Watershed sub-basin: San Juan Islands.
- Larger lakes in San Juan County: Mountain Lake, Cascade Lake, Horseshoe Lake and Spencer Lake.
- Mount Constitution (2,407 feet) is located in Moran State Park on Orcas Island and is the highest point in San Juan County.