Washington State Route 300

State Route 300
Map of Mason County in western Washington with SR 300 highlighted in red
Route information
Auxiliary route of SR 3
Defined by RCW 47.17.540
Maintained by WSDOT
Length:	3.35 mi (5.39 km)
Existed:	1964 – present
Major junctions
West end:	Belfair State Park
East end:	SR 3 in Belfair
Location
Counties:	Mason
Highway system
State highways in Washington Interstate US State Scenic Former PSH 1964 renumbering Former ← SR 292 SR 302 →

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.

SR 300 runs 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. WSDOT has found that more than 11,000 motorists utilize the road daily before the interchange with SR 3 based on average annual daily traffic (AADT) data. SR 300 starts at the entrance to Belfair State Park west of Belfair. 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.

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