The San Juan Islands are a part of the San Juan Archipelago in the northwest corner of the continental United States. The archipelago is split into two groups of islands based on national sovereignty: the San Juans are part of the U.S. state of Washington, while the Gulf Islands are part of the Canadian province of British Columbia.

Prior to European settlement, the islands were part of the traditional area of the Central Coast Salish, which based on language consisted of five groups: Squamish, Halkomelem, Nooksack, Northern Straits (which includes the Lummi dialect), and Clallam. 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.

The U.S.-Great Britain treaty forced by President Polk in 1846 established the 49th parallel as the boundary between Canada and the U.S., except in the San Juan archipelago. While both sides agreed that all of Vancouver Island would remain British, the treaty wording was left vague enough as to put the boundary between modern-day Gulf Islands and San Juan Islands in dispute. Conflicts over this border led to the Pig War in 1859. Skimishes continued until the boundary issue was eventually placed in the hands of the Emperor of Germany for arbitration. The border was finally established in 1872.

Today, the San Juan Islands are primarily a tourist destination, with sea kayaking and killer whale watching two of the primary attractions.

Politically, the bulk of the islands make up San Juan County, Washington, though some of the furthest east of the islands are in the mainland Whatcom County, Washington, most notibly Lummi Island.

The names of the separate islands are, in alphabetical order: Crane Island, Decatur Island, Lummi Island, Obstruction Island, Orcas Island, San Juan Island, Shaw Island, Spieden Island and Waldron Island.