Located just east of Lopez Island across Lopez Sound and just south of Blakely Island across Thatcher Pass, it is 3.524 square miles (9.127 km²) in area, and was named by the Wilkes Expedition in 1841 for naval officer Stephen Decatur.

[1] A post office called Decatur was established in 1891, and remained in operation until 1969.

[3] The San Juan Preservation Trust allows public day-use of their property on the southern tip of the island.

[4] There is no Washington State Ferry service to Decatur; access is by private boat or plane.

Northwest Sky Ferry provides air service from Bellingham, Washington.