The fern-leaved monkeyflower, Mimulus filicifolius (Phrymaceae, Section Simiolus), is a new species described from the northwestern corner of the Sierra Nevada of California. The new taxon is differentiated from close relatives of Mimulus L. (M. laciniatus Gray and M. guttatus DC.) mostly by having many finely divided, bi-pinnately compound leaves. Mimulus filicifolius occurs mainly within ephemeral seeps of rock outcrops, where it occupies similar habitats to M. laciniatus, which occurs farther south in the Sierra Nevada. Mimulus filicifolius appears to be highly geographically restricted, and is currently known only from Butte and Plumas Counties within the Plumas National Forest. It therefore merits strong conservation consideration.