The new genus and species Myrmecicultor chihuahuensis Ramírez, Grismado, and Ubick is described and proposed as the type of the new family, Myrmecicultoridae Ramírez, Grismado, and Ubick. The species is ecribellate, with entelegyne genitalia, two tarsal claws, without claw tufts, and the males have a retrolateral palpal tibial apophysis. Some morphological characters suggest a possible relationship with Zodariidae or Prodidomidae, but the phylogenetic analysis of six markers from the mitochondrial (12S rDNA, 16S rDNA, cytochrome oxidase subunit I) and nuclear (histone H3, 18S rDNA, 28S rDNA) genomes indicate that M. chihuahuensis is a separate lineage emerging near the base of the Dionycha and the Oval Calamistrum clade. The same result is obtained when the molecular data are combined with a dataset of morphological characters. Specimens of M. chihuahuensis were found associated with three species of harvester ants, Pogonomyrmex rugosus, Novomessor albisetosis, and Novomessor cockerelli, and were collected in pitfall traps when the ants are most active. The known distribution spans the Big Bend region of Texas (Presidio, Brewster, and Hudspeth counties), to Coahuila (Cuatro Ciénegas) and Aguascalientes (Tepezalá), Mexico.