Aim To verify the synonymy of the reef-building polychaete Phragmatopoma caudata (described for the Caribbean) and Phragmatopoma lapidosa (described for Brazil) using molecular data. To evaluate the patterns of genetic diversity and connectivity among populations from Florida to South Brazil. Location Intertidal zone in the western Atlantic biogeographical Region: Brazil, eastern Caribbean and Florida (USA). Methods DNA sequence data from one mitochondrial (cox-1) and one nuclear ribosomal (ITS-1) loci were obtained from 11 populations of P. caudata spanning the coasts of Brazil, eastern Caribbean and Florida. Phylogenetic relationships among populations of P. caudata and other members of the genus were inferred by Bayesian methods. Population differentiation was evaluated by Bayesian analysis of population structure (baps), AMOVA and pairwise φst. Demographic history was inferred by Bayesian skyline plots. Results Phylogenetic inference supported the interpretation of a single species of Phragmatopoma spanning the Brazilian and Caribbean Provinces of the western Atlantic Region. Little population structure was observed across the species distribution, with the exception of the Florida population. The baps analysis supported a 2-population model, with population differentiation being strong and significant between Florida and all other Atlantic populations for cox-1, and significant between Florida and most populations for ITS-1. Differences in genetic diversity were not significant between Caribbean and Brazilian populations, although several populations in Brazil had low values for diversity indices. Bayesian skyline plots indicate population expansion starting at c. 200 ka. Main conclusions Phragmatopoma caudata is able to maintain genetic connectivity across most of its geographical range, with population differentiation being observed only between Florida and all other localities, possibly due to ecological speciation in the transition zone between tropical and subtropical environments. Long-distance connectivity across much of the species range is likely the result of long-lived larvae that are tolerant to a wide range of environmental conditions.