Rosario Strait is a strait in northern Washington state, separating Island and San Juan Counties. It extends from the Strait of Juan de Fuca about 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 (just east of Matia 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.