We describe a new species of Opuntia Mill. (Cactaceae), endemic to the region of Sain Alto, from the municipio of Sain Alto, Zacatecas, Mexico. The locals call it zarco or xoconostle (in nahuatl, “xoco” = “acid”; “noxtle” = “tuna”). The new taxon is a tree, its trunk scaly, black in color, with tufts of white short bristles. The first woody branch is parallel to the ground: the epidermis has short trichomes, the areoles of the cladodes are arranged in 16 to 18 series, spines are (three)six to eight, emerging from the lower and lateral sides on all areoles, acicular, 0.6–1.6 cm long, reflexed, the lateral ones horizontal, spines are white in color with an amber base and apex. The flowers are pale yellow to greenish and the pericarpel is pale green, powdery; the fruits are pale pink, the exterior walls very thin, spineless, the inner walls thick, pale pink in color, acidic, and persistent on the cladodes when mature; seeds disposed in the center of the fruit, with dry funiculi, rose in color. The new species is compared with four sympatric species with trichomes on the epidermis and all producing xoconostles: O. guilanchi Griffiths, O. leucotricha DC., O. oligacantha C. F. Först., and O. spinulifera Salm-Dyck. The new species is assigned to Opuntia ser. Leucotrichae Britton & Rose, which includes three species with pubescent epidermis and acidic fruits.