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, six 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. The USGS definition coincides with San Juan County. Islands not in San Juan County are not part of the San Juan Islands, according to the USGS.

Archaeologists use the term "Gulf of Georgia Culture Area" to refer to the San Juan and Gulf Islands, the whole of which shows many archaeological commonalities. The San Juan Islands were part of the traditional area of various peoples of the Coast Salish ethnolinguistic group. Linguistically, Coast Salish groups in the area consist of the Nooksack, Northern Straits (which includes the Lummi, Klallam, Saanich, and Songhees dialects. Exploration and settlement by Europeans brought smallpox to the area by the 1770s. In 1843, the Hudson's Bay Company established Fort Camosun at nearby Vancouver Island.

San Juan (/ˌsæn ˈhwɑːn/; Spanish pronunciation: [saŋ ˈxwan], Saint John), officially Municipio de la Ciudad Capital San Juan Bautista (Municipality of the Capital City, Saint John the Baptist), is the capital and most populous municipality in Puerto Rico, an unincorporated territory of the United States. As of the 2010 census, it had a population of 395,326 making it the 46th-largest city under the jurisdiction of the United States. San Juan was founded by Spanish colonists in 1521, who called it Ciudad de Puerto Rico ("Rich Port City"). Puerto Rico's capital is the second oldest European-established city in the Americas, after Santo Domingo, in the Dominican Republic. Several historical buildings are located in San Juan; among the most notable are the city's former defensive forts, Fort San Felipe del Morro and Fort San Cristóbal, and La Fortaleza, the oldest executive mansion in continuous use in the Americas.

Today, San Juan is one of Puerto Rico's most important seaports, and is the island's manufacturing^{[citation needed]}, financial, cultural, and tourism center. The population of the Metropolitan Statistical Area, including San Juan and the municipalities of Bayamón, Guaynabo, Cataño, Canóvanas, Caguas, Toa Alta, Toa Baja, Carolina and Trujillo Alto, is about 2 million inhabitants; thus, about half the population of Puerto Rico now lives and works in this area. San Juan is also a principal city of the San Juan-Caguas-Fajardo Combined Statistical Area. The city has been the host of numerous important events within the sports community, including the 1979 Pan American Games, 1966 Central American and Caribbean Games, 2006 and 2009 World Baseball Classics, the Caribbean Series and the Special Olympics and MLB San Juan Series in 2010.