The opisthobranch mollusks recorded in the National Park Arrecife Alacranes, a marine protected area and the largest coral reef in the southern Gulf of Mexico, were studied during three survey periods in 2009-2010. Several substrate types: rocks, sponges, corals, sand, Thalassia beds and drifting Sargassum were analyzed. A total of 32 species, belonging to 22 genera, 15 families and four orders were registered. The most diverse order was the Nudibranchia, probably due to the variety of habitats and the feeding resources they exploit. The species Elysia crispata, E. tuca, Aplysia dactylomela, Phyllaplysia engeli and Phidiana lynceus were recorded during the three sampling seasons, suggesting that they are common in the reef system. The ecological success of the Elysia species is due to their ability of incorporate and maintain chloroplasts photosynthetically active, providing the species with extra energy in the case of limited feeding resources. Our own and previous records of Elysia cf. timida, Ercolania cf. coerulea, Polycera herthae, Aeolidiella stephanieae, Spurilla sargassicola, Doto pygmaea and Learchis evelinae suggested that these organisms are not frequent species in the western Atlantic. Finally, the geographical range of S. sargassicola is extended southwards, and that of L. evelinae towards north.