Euphorbia rayturneri V. W. Steinm. & Jercinovic (Euphorbiaceae), a new species endemic to southwestern New Mexico, United States, is described and illustrated. This summer annual occurs in desert grassland and is known from three collections made in Grant, Hidalgo, and Luna counties at elevations of 1400–1700 m. Given its limited distribution and small population size, we propose that the species is best considered as CR, or Critically Endangered, according to IUCN Red List criteria. The new species belongs to Euphorbia L. sect. Anisophyllum Roep. on the basis of possessing interpetiolar stipules and opposite leaves that are asymmetrical at the base. It is characterized by being a pubescent, prostrate herb with serrulate leaves that often have a red blotch in the center and an ovary with undivided styles.