Rosario Strait is a strait in northern Washington state, separating San Juan County and Skagit and Whatcom Counties. It extends from the Strait of Juan de Fuca about 23 kilometres (14 mi) north to the Strait of Georgia. The USGS defines its southern boundary as a line extending from Point Colville on Lopez Island to Rosario Head on Fidalgo Island, and its northern boundary as a line from Point Migley on Lummi Island to the east tip of Puffin Island and then to Point Thompson on Orcas Island. Rosario Strait runs north-south between Lopez, Decatur, Blakely, and Orcas Islands on the west, and Fidalgo, Cypress, Sinclair, and Lummi Islands on the east.

San Juan County is a county in the Salish Sea in the far northwestern corner of the U.S. state of Washington. As of the 2020 census, its population was 17,788. The county seat and only incorporated town is Friday Harbor, on San Juan Island. The county was formed on October 31, 1873, from Whatcom County and is named for the San Juan Islands, which are in turn named for Juan Vicente de Güemes, 2nd Count of Revillagigedo, the Viceroy of New Spain.