Aphyllon epigalium Colwell & A.C. Schneid. is described as a new species from Oregon and California. This taxon is distinguishable from the other members of Aphyllon Mitch. sect. Aphyllon in Western North America (i.e., A. fasciculatum [Nutt.] Torr. & A. Gray, A. purpureum [A. Heller] Holub, and A. uniflorum [L.] Torr. & A. Gray) in its host preference for Galium L., by having 2–4 yellow flowers per stem, and pedicels longer than the stem. Two subspecies are described: Aphyllon epigalium subsp. epigalium, which occurs in the Cascade and Klamath ranges in Oregon and California, and the Coast Ranges and Sierra Nevada in California; and A. epigalium subsp. notocalifornicum Colwell & A.C. Schneid., currently known from few sites in montane southern California. The new subspecies differ from one another in flower size, corolla lobe shape, host preference, geographic range, and nuclear and plastid genetic markers. An updated key to California and Oregon Aphyllon sect. Aphyllon is presented.