Washington State Route 121

State Route 121
SR 121 highlighted in red
Route information
Auxiliary route of US 12
Maintained by WSDOT
Length	7.67 mi[1] (12.34 km)
Existed	1967[2]–present
Major junctions
South end	I-5 near Maytown
North end	I-5 near Tumwater
Location
Country	United States
State	Washington
Counties	Thurston
Highway system
State highways in Washington Interstate US State Scenic Pre-1964 1964 renumbering Former
← SR 119 → SR 122

State Route 121 (SR 121) is a state highway located entirely in Thurston County, Washington, United States. The highway serves Millersylvania State Park, forming a 7.67-mile (12.34 km) loop between two interchanges with Interstate 5 (I-5) near Maytown. It is an auxiliary route of U.S. Route 12 (US 12).

The highway follows roads constructed in the early 20th century and maintained by the Thurston County government until their incorporation into the state highway system. The state government created Secondary State Highway 1M (SSH 1M) in 1937, initially connecting Rochester to Tumwater before its north end was truncated to Maytown in 1959. SSH 1M was renumbered in 1964 to the short-lived SR 801, which was then replaced by SR 121 in 1967. The Rochester–Maytown section of SR 121 was replaced by the route through Millersylvania State Park in 1991, severing its connection to US 12.