Chorizanthe aphanantha K.M.Nelson & D.J.Keil is described as a new species from serpentine soils in the Irish Hills portion of the San Luis Range of central-western San Luis Obispo County. It occurs in portions of San Luis Obispo's Irish Hills Natural Reserve, where it grows with several other rare, serpentine-endemic taxa. It is distinguished from C. breweri S.Watson, with which it co-occurs and may be confused (particularly as it ages), by its compact, rather than sprawling habit; the green rather than red color of its herbage at the time of flowering; its much smaller, white, monomorphic perianth lobes, rather than white to pink, dimorphic lobes; bracts with straight, ascending, rather than recurved spine tips; smaller and smooth rather than transversely corrugated mature involucres; and basal leaf blades that are glabrous to minutely strigose, thin, and ovate to elliptic (occasionally mucronate) with obscure veins, rather than thick, leathery, reniform, and apically notched to ±obcordate with pronounced veins. It differs from the recently described C. minutiflora R.Morgan, Steyer, & Reveal by its sparser and shorter pubescence, entire versus erose perianth lobes, nine rather than three stamens, ovate to elliptic rather than oblanceolate basal leaves, and flat rather than undulating leaf margins. It is distinguished from the morphologically similar C. procumbens Nutt. by its ovate to elliptic rather than oblong to oblanceolate leaf blades, its sparser and shorter pubescence, the presence of a narrow scarious margin on the involucral teeth, smaller flowers, and distinct rather than basally connate filaments. A key is provided that distinguishes C. aphanantha from other Chorizanthe species in the San Luis Range. It is currently known to grow in serpentine scrub and chaparral at elevations from 100 to 370 meters.