Polychaete annelids in the genus Hermodice exhibit an amphi-Atlantic distribution extending into the Gulf of Mexico as well as the Caribbean and Mediterranean Seas. Currently, this genus contains two nominal species; H. carunculata, which inhabits the Greater Caribbean, and H. nigrolineata which is found in the Mediterranean and along the African Atlantic coast. In this study, we analyzed DNA sequence data from specimens in the Gulf of Mexico (GoM), the Mediterranean and Caribbean Seas to estimate population structure and phylogeographic patterns for Hermodice. Analysis of mitochondrial cytochrome oxidase subunit I (COI) and 16s rDNA indicates low levels of divergence between Greater Caribbean and Mediterranean specimens. Additionally, at least three outliers from Mediterranean populations cluster with Greater Caribbean specimens in phylogenetic analyses. Our results provide no support for the distinction between H. carunculata and H. nigrolineata and stress the importance of including molecular data as a criterion for delimiting species. Whereas other genetic studies of marine taxa often reveal the presence of cryptic species complexes, our study indicates high population connectivity across a wide geographic range for H. carunculata.