The San Juan Islands are an archipelago in the northwest corner of the contiguous United States between the US mainland and Vancouver Island, British Columbia, Canada. The San Juan Islands are part of the U.S. state of Washington.

In the archipelago, four islands are accessible by passenger ferry operated by the Washington State Ferries system.

The United States Geological Survey (USGS) defines the San Juan Islands as the archipelago north of the Strait of Juan de Fuca, west of Rosario Strait, east of Haro Strait, and south of Boundary Pass. To the north lie the open waters of the Strait of Georgia. All these waters are within the Salish Sea. The USGS definition of the San Juan archipelago coincides with San Juan County. Islands not in San Juan County are not part of the San Juan Islands, according to the USGS.