State Route 300 (SR 300) is a short state highway in Mason County, in the U.S. state of Washington. It extends 3.35 miles (5.39 km) from Belfair State Park to SR 3 in the community of Belfair. The route serves as a connector from Belfair State Park to Belfair and SR 3. The highway was Secondary State Highway 21C (SSH 21C) from 1957 until 1964.