It stretches from the western terminus of the 49th parallel segment of that boundary, and was chosen by the arbitrator in the San Juan Islands dispute (Pig War) over the Rosario Strait.

[1] Haro Strait and other waters flanking the Gulf Islands and San Juans were the home of Straits Salish peoples including the Lummi, Saanich, and Klallam, but the route's natural importance as a regional waterway made it also an important sea-route for raiding and also for regular trade for all marine peoples of the Northwest Coast.

[3] Haro Strait's status as the location of the international boundary was not established until the resolution of the San Juan Islands dispute in 1870, when it was selected by an arbitrator, German Kaiser Wilhelm I, over Rosario Strait, on the eastern side of the San Juans, which was preferred by the British and would have made the San Juans part of British Columbia, as they were originally viewed to be by the British after the Oregon Treaty of 1846.

D'Arcy Island on the Canadian side of the strait was a leper colony for Chinese immigrants in the 19th century.

The strait is a favourite locale for whale-watching tours based in Greater Victoria and also in the San Juans because of the density and variety of its marine life, including orcas (killer whales), sea lions, seals and a large range of different species of sea birds.