Lagophylla diabolensis is a new hare-leaf from the southern Diablo Range of Fresno, Monterey, and San Benito counties, California. Plants of the Diablo Range hare-leaf were previously included within L. dichotoma, which is treated here in a restricted sense to comprise plants from the Sierra Nevada foothills and eastern Great Central Valley. Lagophylla diabolensis differs morphologically from L. dichotoma by having consistently glandular distal foliage (glands clear to dark-purple), narrower cauline leaves, generally uniformly tawny stems, and smaller heads. The taxonomic significance of those morphological differences is corroborated by other evidence that L. diabolensis is more closely related to the widespread L. ramosissima than to L. dichotoma sensu stricto. The Diablo Range hare-leaf occurs as scattered colonies, often in clayey soils of grassy openings in oak-pine woodland below 1100 m elevation. Extreme rarity and paucity of recent collections of L. diabolensis and L. dichotoma in the current sense indicates that both species warrant conservation concern.