A new species of theRhadinaeadecoratagroup is described based on two specimens from the Sierra Madre del Sur, Guerrero, Mexico. The new species differs from all other members of the genusRhadinaeaby having: eight supralabials; 149–151 (male) ventrals; 63–77 (male) subcaudals; two large pale nuchal blotches, forming an incomplete collar that occupies two scales laterally and is bissected along the dorsal midline; a postocular pale marking consisting of a well-defined, narrow line beginning behind the upper posterior margin of the eye and extending posteriorly nearly horizontally until connecting with the nuchal blotches; and the dark ground color of the flanks extending to the lateral portion of the ventrals. The large nuchal blotches distinguish the new species from the other members of theR.decoratagroup, except forR.cuneataand some individuals ofR.hesperia(pale nuchal marking one-scale wide inR.marcellae, absent in the other species). The condition of the postocular pale marking distinguishes it fromR.cuneataandR.hesperia(postocular pale marking wedge-shaped inR.cuneata, not connected with the pale post-cephalic markings inR.hesperia). Furthermore, the number of subcaudals and the coloration of the lateral portion of the ventrals distinguish it fromR.omiltemanaandR.taeniata, the remaining congeners found in Guerrero (85–90 in males ofR.omiltemanaand 91–121 inR.taeniata; dark color of the flanks not reaching ventrals in the former species, occasionally and faintly inR.taeniata). Additionally, a new combination forR.stadelmaniis proposed. The new species is the first described in the genusRhadinaeain more than 40 years.
