SR 300 runs 3.35 miles (5.39 km) from Belfair State Park to SR 3 in the community of Belfair.

[3][4] WSDOT has found that more than 11,000 motorists utilize the road daily before the interchange with SR 3 based on annual average daily traffic (AADT) data.

From the state park, the highway goes northeast along the coastline of the Hood Canal to Belfair, and turns south to merge onto SR 3.

[3][6] When the Primary and Secondary Highways were realigned in 1957, the current SR 300 became Secondary State Highway 21C (SSH 21C).

[7] SSH 21C became SR 300 in 1964 during the 1964 highway renumbering, in which the Washington State Department of Transportation (WSDOT) replaced the previous system of Primary and Secondary Highways with a new system called State Routes, which is still in use today.