Cryptantha martirensis M. G. Simpson & Rebman is described as new, being endemic to high elevations of the Sierra de San Pedro Mártir of Baja California, Mexico. It is sparse to common in the understory of coniferous woodland and in montane arroyos, slopes, and ridges in sandy to gravelly granitic substrates. This new species is similar to C. muricata (Hooker & Arnott) A. Nelson & J. F. Macbride in having nutlets with a shallow, dorsal ridge. It differs from the three recognized varieties of C. muricata in having a combination of tall and virgate primary stems with short and clustered inflorescence units; stems with mostly appressed and a few sparse, fine, spreading trichomes; a small corolla limb; and relatively large nutlets with dorsal tubercles that are low, rounded, and few per area. Quantitative evidence justifying these differences is summarized.