Sidalcea gigantea G. Clifton, R. E. Buck, & S. R. Hill is described as a new species from the northwestern slope of the Sierra Nevada in Butte, Plumas, Sierra, and Yuba Counties and from the extreme southern Cascade Range in Shasta County, California. The new species is a robust, long-lived perennial from large and extensive rhizomes; it is one of the tallest species known in the genus and it forms the largest known clonal colonies. Based on both morphological and DNA characters, S. gigantea appears to be most closely related to S. asprella Greene and S. celata (Jeps.) S. R. Hill, both found in the vicinity of the new species. Sidalcea gigantea occurs around margins of meadows, seeps, and streams in montane conifer forests, especially Mixed Conifer Forest.