AbstractThe marine area of Área de Conservación Guanacaste (ACG) contains a 43,000 ha formal marine protected area, a 732 ha special management zone in Bahía Santa Elena, and 150 km of wild protected coastline. In an effort to broaden the biodiversity knowledge of all marine taxa present in the area, an inventory was started in 2015 (BioMar-ACG Project). This initiative is being funded by the Guanacaste Dry Forest Conservation Fund (GDFCF) in collaboration with government staff at ACG, and is carried out by Centro de Investigación en Ciencias del Mar y Limnología (CIMAR, Center for Research in Marine Science and Limnology) and Museo de Zoología (Zoology Museum), both from Universidad de Costa Rica (UCR). After four years of the project, 2650 specimens of marine decapod crustaceans and stomatopods have been collected, belonging to 209 species, out of which 99 are new records for ACG, four may be new species and nine (Cyrtoplax panamensis, Glyptoxanthus labyrinthicus, Pachyches marcortezensis, Petrolisthes donadio, Pylopagurus holmesi, Synalpheus pinkfloydi, Typton granulosus, Zenopontonia soror, Neogonodactylus pumilus) are new records for Costa Rica. With this contribution the total number of decapods (257 spp.) and stomatopods (14 spp.) for ACG is 271 species, more than half the species reported for Costa Rica, and more than a quarter of all crustaceans reported for the eastern tropical Pacific. The high concentration of species in ACG may be attributed to the diversity of habitats, the seasonal upwelling and to the recent sampling efforts. In only four years, the BioMar-ACG has increased the number of species in these groups of crustaceans by 37% over the past 85 years of previous studies in the ACG.