Whatcom County is a county located in the northwestern corner of the U.S. state of Washington, bordered by the Lower Mainland of British Columbia to the north, Okanogan County to the east, Skagit County to the south, San Juan County across Rosario Strait to the southwest, and the Strait of Georgia to the west. Its county seat and largest population center is the coastal city of Bellingham, comprising the Bellingham, WA Metropolitan Statistical Area, and as of the 2020 census, the county's population was 226,847.

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.