A female of the monstrilloid copepod species Cymbasoma tumorifrons (Isaac, 1975), previously known only from the Aegean Sea, was recorded in a bay of the Mexican tropical Pacific. The female of this rare species has not been recorded since its description more than 25 yr ago. To complement the brief original description, an expanded description and illustrations of this species are presented here based mainly on the Mexican specimen but considering also type material. Standards recently set for description of monstrilloid copepods were followed. Several characters such as chaetotaxy of legs 1-4, details of the antennular armature, cuticular ornamentation, and the fifth legs were overlooked in the original diagnosis and are described and analyzed here. This record supports an important range extension of the known distribution of this species from the Mediterranean Sea to the Mexican Pacific. It represents only the fourth record of a Cymbasoma species in the eastern Pacific.