Update of the catalog of opisthobranch mollusks (Gastropoda: Heterobranchia) from the Catalan Coasts.¿An extension of the checklist of opisthobranch species (Gastropoda: Heterobranchia) known from the Catalan Coasts is presented, based on numerous unpublished findings by the authors and reports confirmed with pictures posted in several Internet platforms. A total of 53 species are added to the previous catalog: 4 Cephalaspidea, 9 Runcinacea, 3 Anaspidea, 8 Sacoglossa and 29 Nudibranchia (11 Doridacea, 10 Aeolidacea, 2 Dendronotacea and 6 Cladobranchia incertae sedis). Data for the different reports of each of these species are provided, along with some biological, distribution or taxonomical remarks of interest. Finally, an updated and taxonomically sorted list of all opisthobranch species known for Catalonia is provided, including a total of 257 species, of which 9 are basal Heterobranchia, 35 Cephalaspidea, 13 Runcinacea, 10 Anaspidea, 22 Sacoglossa, 8 Pleurobranchomorpha, 2 Umbraculida, 3 Gymnosomata, 11 Thecosomata and 144 Nudibranchia (66 Doridacea, 51 Aeolidida, 12 Dendronotida, 12 Cladobranchia incertae sedis and 3 Euarminida). With all the new data reported in this paper, and regarding opisthobranch mollusks, the Catalan Coast becomes the biologically most diverse geographical region in the Iberian Peninsula.