The northern limit of its range is somewhat north of the border between Canada and the United States in British Columbia.

It is less abundant in the coastal mountains of Washington and northern Oregon and more common south of the Siuslaw River.

It is mostly coastal in distribution but extends its range eastward to locations in the Cascade Mountains in Oregon, Washington and British Columbia.

[6] Archibald Menzies found R. macrophyllum growing along with Arbutus menziesii in May 1792 when he and George Vancouver made their second landfall after leaving Hawaii, near present-day Port Discovery, Washington.

[8] The WNARSP is documenting the detailed range and forms of all of the western North American rhododendron species.