We herein describe a new species of Cathariotrematinae Bullard, 2021 (Monogenoidea: Monocotylidae Taschenberg, 1879) and propose a new genus to accommodate it. Nasigulper itsgotatao n. gen., n. sp. infects the olfactory lamellae (nose) of the deepwater gulper shark, Centrophorus granulosus (Bloch and Schneider, 1801) (Squaliformes: Centrophoridae) from the north-central Gulf of Mexico outer continental shelf. The new species was assigned to Cathariotrematinae by having a 3-part attachment organ (TAO) and by lacking open loculi that symmetrically encircle a cluster of >2 loculi in the center of the haptor. The proposal of a new genus is warranted because the new species is the only known monocotylid that has a TAO comprising 3 closed loculi with 2 relatively small hamuli flanking the posteriormost loculus. The most morphologically similar cathariotrematine genus is represented by a single species (Squalotrema llewellyni Kearn and Greene, 1983) that infects the spiny dogfish, Squalus acanthias (Squaliformes: Squalidae) in the North Sea. Nasigulper itsgotatao differs from Squalotrema llewellyni by having a TAO comprising 3 closed loculi, relatively small hamuli flanking the posteriormost loculus, and a relatively short, sinous ovovitelline duct and by lacking depressions associated with the central loculus. Squalotrema llewellyni has a TAO comprising 3 supraloculi, hamuli that are equal to the length of the posteriormost loculus, a central loculus having a central depression flanked by 2 pear-shaped depressions anterolaterally, and an extensive ovovitelline duct that is sinuous sinistral to the proximal ovary. A phylogenetic analysis of all acceptable monocotylid 28S sequences recovered the new species sister to isolates of Cathariotrema selachii (MacCallum, 1916) Johnston and Tiegs, 1922 from the Gulf of Mexico (no sequence of a species of Squalotrema exists). The new genus and species are the sixth and eighth, respectively, added to Cathariotrematinae.