Trillium scouleri is a species of flowering plant in the bunchflower family Melanthiaceae.
In both habit and flower structure, Trillium scouleri closely resembles Trillium ovatum. To distinguish the two species, the only reliable characters are the lengths of the filaments and anthers. In absolute terms, the filaments and anthers of T. scouleri are almost always longer than the corresponding characters in T. ovatum. The minimum, average, and maximum lengths of the filaments and anthers of each species is as follows:
| Filaments | Anthers | ||||||
|---|---|---|---|---|---|---|---|
| Min | Ave | Max | Min | Ave | Max | ||
| T. scouleri | 5 | 6.4 | 9 | 8 | 10.8 | 15 | |
| T. ovatum | 4 | 4.4 | 5 | 5 | 6.4 | 7 | |
The relative lengths of the filaments and anthers are also significant. In T. ovatum, the length of the filaments is equal to (or slightly less than) the length of anthers. In T. scouleri, the length of the filaments is one-half to two-thirds the length of the anthers.
Trillium scouleri was described by the American ecologist, botanist, and taxonomist Henry A. Gleason in 1906. Gleason included the Swedish-born, American botanist Per Axel Rydberg as a co-author but cited no reference. In any case the correct name of the taxon is Trillium scouleri .
Gleason's description is based on some notes published by the English botanist William Jackson Hooker in 1838. One of the specimens examined by Hooker was evidently collected by David Douglas and John Scouler while serving on board a Hudson's Bay Company ship that sailed from London in 1824. The ship was bound for the Columbia River in what is now known as the Pacific Northwest region of North America. Presumably the specimen was collected there circa 1825, which disagrees with Gleason's claim that the type specimen of Trillium scouleri was collected in British Columbia.
For many decades, Trillium scouleri was assumed to be a synonym for Trillium ovatum, but more recently it was accepted as a distinct species based on unpublished molecular evidence., Trillium scouleri is widely accepted by authorities.