Cucullanus mycteropercae n. sp. is described from the intestine of the black grouper, Mycteroperca bonaci Poey, from the northern coast of Yucatán, México. The new species is readily distinguishable from other Cucullanus species because it possesses an ellipsoidal papilla-like structure situated medially on the anterior cloacal lip of males. Other differentiating characters include the variable position of postcloacal pair 8 in males, the subventral position of phasmids, a slightly ventrally hooked posterior end of gubernaculum, and the presence of a large, cylindroconical sclerotized tail end in both sexes. This is the fourth record of a marine cucullanid off the Yucatán Peninsula in México belonging to Cucullanus Müller, 1777.