Streptanthus longisiliqus G. Clifton & R. Buck is described as a new species from Butte, Tehama, and Shasta counties in northeastern California. The new species is a short-lived perennial in subgenus Pleiocardia, informal group Cordati. Based on several morphological characters, S. longisiliqus appears to be most closely related to the montane southern California species S. bernardinus and S. campestris; it is geographically disjunct from those taxa by approximately 600 km. Streptanthus longisiliqus occurs in montane coniferous forest, often in localized openings, and sometimes in disturbed places.