State Route 21 (SR 21) is a 191.34-mile-long (307.93 km) state highway in the U.S. state of Washington that traverses four counties: Franklin, Adams, Lincoln and Ferry. The highway extends from an intersection with SR 260 in Kahlotus north through Lind, Odessa, Clark, Keller, Republic and Curlew before becoming Highway 41 (BC 41) at the Canadian border in Danville. SR 21 is concurrent with U.S. Route 2 (US 2) in Wilbur and SR 20 in Republic and has two diamond interchanges at US 395 in Lind and Interstate 90 (I-90) south of Odessa. Between Lincoln and Ferry counties, the roadway crosses Franklin D. Roosevelt Lake on the Keller Ferry, operated fare free by the Washington State Department of Transportation (WSDOT) and the Department of Highways (DoH) since 1930.

The Keller Ferry or Clark Ferry, is a ferry crossing on Franklin D. Roosevelt Lake in the US state of Washington. The crossing carries State Route 21 between the Colville Indian Reservation in Ferry County and Clark in Lincoln County. The ferry has been in operation since the 1890s and under state control since 1930. During that time, five vessels have served the crossing, including the Martha S., which operated from 1948 to 2013, and the current ferry, the M/V Sanpoil.