We describe a new helicoidean semi-slug based on morphological and molecular evidence. The new species belongs to the genus Bunnya and is described from a small agricultural area in Zinacantepec, San Juan de las Huertas, México. The genus Bunnya is externally similar to Xanthonyx, another Helicoidea genus; both genera have similar shell and body form, and both have a tail horn. Internally, Bunnya is similar to Humboldtiana, since both have dart-sacs (3 in Bunnya, 3−4 in Humboldtiana) surrounding the vagina, two dart-bulbs associated with each dart sac, and a gland number similar to the dart-sacs number. Four adult specimens of Bunnya metli n. sp. were dissected and compared with the two described species: B. bernardinae from Cuajimalpa, México City, and B. naranjoe from Sierra de Manantlan, Jalisco. Bunnya metli n. sp. is characterized by: very closely-spaced radial riblets (about 23 per mm) crossed by fine spiral wavy threads on the embryonic whorls; unfused glands inserted on the vagina above the dart-sacs and only one dart per sac; a short, swollen penis with a large spherical verge about half the penis size; an elongated bursa copulatrix with a slight constriction in the middle; a long bursa copulatrix duct; and dart sacs with a muscular pad within the vaginal lumen, surrounding the basal part of the genitalia. We performed a phylogenetic analyses using data from fragments of 16S rRNA mtDNA and 28S rRNA genes from one of the collected specimens, as well as other helicoidean sequences from GenBank. Our analyses support the membership of Bunnya metli n. sp. in Xanthonychidae. This suggests that the taxonomic position of Bunnya in Humboldtianidae or Xanthonychidae warrants reevaluation.