Brodiaea sierrae (Themidaceae), a new diploid species from the Sierra Nevada foothills of California, U.S.A., is described and illustrated. Morphologically, it is most similar to the tetraploid B. leptandra, from which it differs by having larger flowers, with perianth lobes up to 40 mm long (vs. 20–30 mm). The new species is also morphologically similar to B. californica but differs by the staminode morphology and by the lack of anther appendages. Although morphology and other data support a close relationship between the three species, they are geographically and edaphically disjunct in northern California.