A new marine leech is herein described from specimens infecting the external surfaces, including the mouth and cloaca, of the banded guitarfish, Zapteryx exasperate, captured in the Gulf of California and eastern Pacific Ocean off the coast of San Diego, California. The leech is assigned to Austrobdella by possessing continuous contractile coelomic channels that lie outside the somatic musculature along the lateral edges of the urosome (marginal lacunae), clitellar gland cells densely packed in the urosome, 5 pairs of testisacs, and 6-annulate mid-body somites. The new leech is distinguished from its 6 congeners on the basis of body size (maximum 10 mm long) and shape (sub-cylindrical trachelosome distinctly demarcated from wider urosome that is ventrally flattened, convex dorsally, and narrowing toward caudal sucker that is narrow, 20−25% of maximum body width), number of eyespots (2 pairs), shape and arrangement of the ovisacs (pyriform and limited to somites XII/XIII), and characteristics of the midgut (1 pair of mycetomes, 6 pairs of simple thin-walled crop ceca, ventral postceca wanting, and 2 pairs of dendritic diverticula emerging from anterior portion of thick-walled intestine). The new species occurs in the northeastern Pacific Ocean on a benthic elasmobranch. Examination of host specificity for each Austrobdella species using the quantitative Index of Phylogenetic Host Specificity revealed that the new species is 1 of 4 oioxenous specialists in the genus, and the remaining 3 congeners are relative generalists herein classified as euryxenous. This is the first time host specificity for members of the Piscicolidae has been quantitatively assessed. The analysis suggests that associations between marine leeches belonging in Austrobdella and their vertebrate hosts are driven by ecological influences rather than host taxonomic placement.