The type of the species is based on a collection made in Big Canyon, Wallowa County (Oregon), which was found by botanist Edmund Perry Sheldon in 1896.

The specimen was labeled as "enteromorpha", but as the authors explain, that name had been "applied indiscriminately to vitally every nonseridiate Hypogymnia species in North America at some time.

[2] The thallus of Hypogymnia occidentalis, usually tightly pressed to its substrate, is made of small lobes (typically less than 3 wide).

[1] Hypogymnia rugosa is somewhat similar in appearance to H. occidentalis, but the former has broader lobes, does not have side lobules, and contains hypoprotocetraric acid.

It often grows on the lower trunks of conifers, particularly Douglas-fir, and is found at elevations ranging from sea level up to 1,500 m (4,900 ft).