The taxonomic study of monstrilloid copepods is hampered by incomplete early descriptions, uncertain synonymies, and the difficulty of reliably matching males and females of species. A re-evaluation of male monstrilloid specimens collected from two reef areas of the Mexican Caribbean allowed me to clarify the status of Monstrilla mariaeugeniae Suárez-Morales & Islas-Landeros, 1993 and M. wandelii Stephensen, 1913 based on a comparison of males attributed to each of these species. Males from the Puerto Morelos reef system, northern Mexican Caribbean coast, were first proposed as a tropical subspecies of the subarctic M. wandelii; later on, morphologically close males collected from the Mahahual reef area, southern Mexican Caribbean coast, were designated as the males of M. mariaeugeniae. Their status is here corrected with the description of M. mahahualensissp. nov. based on the Mahahual males; the new species shares the same type of genitalia with the antarctic M. conjunctiva Giesbrecht, 1892 and the subarctic M. wandelii; Park (1967) linked a single male from Vancouver to M. wandelii. It was realised that Park’s (1967) males from the Vancouver area and the two Mexican Caribbean groups of males represent different, undescribed species. I here reassign the males earlier attributed to M. mariaeugeniae as a new species of Monstrilla which is herein described. The new species differs from the males of M. conjunctiva and M. wandelii by details of the genitalia, length of the setae of the fifth legs, armature and integumental structures of the antennules, and size of the outer exopodal spines of legs 1–4. This is the third known species of Monstrilla with a M. conjunctiva-like male genitalia and the first one known from tropical areas. The male of both M. mariaeugeniae and M. wandelii remain unknown.