Belfair is a census-designated place in Mason County, Washington, United States. Located at the mouth of the Union River at Hood Canal, it serves as the commercial center of northern Mason County. The population of the surrounding area grows in the summertime, as the Canal and the Olympic Peninsula are popular with tourists. The population was 3,931 as of t…Belfair is a census-designated place in Mason County, Washington, United States. Located at the mouth of the Union River at Hood Canal, it serves as the commercial center of northern Mason County. The population of the surrounding area grows in the summertime, as the Canal and the Olympic Peninsula are popular with tourists. The population was 3,931 as of the 2010 census.