Birch Bay is a protected bay of the east shore of the Salish Sea, between Semiahmoo Bay and Lummi Bay; approximately 100 miles north of Seattle and 35 miles south of Vancouver, BC, Canada. The bay gives its name to a census-designated place along the bay's shore in Whatcom County, Washington, United States. The population was 8,413 at the 2010 census, a 69.6% increase over 4,961 the 2000 census. As of the 2020 census, the population was 10,115.