Eurythoe complanata was considered a circumtropical species, occurring in the Atlantic, Pacific and Indian oceans and the Mediterranean and Red seas. However, a recent study by Barroso et al. (2010) looking at morphological features, allozyme analyses, DNA analysis and molecular divergence suggests that the true range of E. complanata is limited to the Atlantic Ocean, specifically from the Caribbean Sea to southern Brazil. Worms found on the coasts of Atlantic islands often show distinct morphological features of the parapodia, leading to debates about the species status. Most classifications include the coastal Atlantic island worms since the differences are small and generally resemble variations found in any species. Polychaetes identified as E. complanata around the globe actually consist of three or more closely related species sharing many morphological traits.

Biogeographic Regions: oceanic islands ; atlantic ocean