The genus Promicrogaster (Hymenoptera, Braconidae, Microgastrinae) from Area de Conservación Guanacaste (ACG), Costa Rica, is revised. A key is provided to all new species as well as those previously described from Mesoamerica. A total of 21 species, all authored by Fernandez-Triana & Boudreault, are described as species nova: alexmartinezi, andreyvallejosi, brandondinartei, daniellopezi, daretrizoi, eddycastroi, eimyobandoae, fabiancastroi, fabriciocambroneroi, hillaryvillafuerteae, kevinmartinezi, kiralycastilloae, leilycastilloae, liagrantae, luismendezi, monteverdensis, naomiduarteae, pablouzagai, ronycastilloi, sebastiancambroneroi, tracyvindasae. A species previously described from India is considered as incertae sedis. Promicrogaster is considered to be restricted to the New World, with the vast majority of the species found in the Neotropics and a few extending north to the Nearctic. Almost 60% of the known species in ACG are found in cloud forests at over 1,000 m altitude. All of the verified and authenticated host records for Promicrogaster are from caterpillars living more deeply inside plant tissue than simply in rolled leaf structures – although no host data from ACG is available.