The taxonomy of flabelligerid polychaetes has been difficult because of three main problems: 1) the eversible anterior end is rarely exposed and its appendices are poorly known, 2) there is no standard terminology for chaetae, and 3) most generic definitions have been unstable or improperly defined. A redefinition and revision of all species in Piromis Kinberg, 1867 and Pycnoderma Grube, 1877 is herein presented. Piromis, Pycnoderma, and Trophoniella Caullery, 1944 share a thick tunic and a projected, tongue-shaped branchial plate. Their main difference is the type of neurochaetae in median and posterior chaetigers; thus, Piromis has multiarticulate, often bidentate neurochaetae, whereas Pycnoderma has oligoarticulate, often mucronate neurochaetae, while Trophoniella has anchylosed neurochaetae. This latter genus is being revised elsewhere. The species in Piromis or Trophoniella are separated depending on the extent and type of sediment cover, together with the presence of notopodial or dorsal lobes, the relative length of chaetal articles, especially in neurochaetae, and the relative abundance of body papillae or cephalic cage chaetae. As herein defined, Piromis includes 12 species with five being newly described, while Pycnoderma includes eight species with three being newly described. The species in Piromis are P. arenosus Kinberg, 1867 (type species), P. amoureuxi n. sp. from the Lesser Antilles, P. brisegnoi n. sp. from the Gulf of California, P. capulata (Moore, 1909) n. comb., P. eruca (Claparede, 1869), P. fauchaldi n. sp. from the Gulf of California, P. kisemboanus (Augener, 1918) n. comb., P. robertsi (Hartman, 1951), P. suni n. sp. from the South China Sea, P. vossae n. sp. from the Strait of Florida, P. websteri Day, 1973 n. status, and P. wehei n. sp. from the Indian Ocean. On the other hand, the species in Pycnoderma are P. congoense Grube, 1877 (type species), P. dannyi n. sp. from tropical Western Africa, P. escobarae n. sp. from the Gulf of Mexico, P. ferruginea (Gallardo, 1968) n. comb., P. glabra (Treadwell, 1901) n. comb., P. glasbyi n. sp. from Northeastern Australia, P. gracilis (Hartman, 1961) n. comb., and P. talehsapensis (Fauvel, 1932) n. comb. Another species, P. fernandense Augener, 1918, described from two localities from Ghana and Equatorial Guinea, is transferred to Trophoniella Caullery, 1944, by having anchylosed neurospines in posterior chaetigers. All Piromis and Pycnoderma species thrive in sediments, sometimes in mixed environments, in tropical to temperate regions. Piromis includes mostly shallow water species while Pycnoderma includes species living in greater depths.