The species diversity of Syllidae (Annelida: Polychaeta) on different coastlines withinthe Mediterranean Basin was analysed, including along the Salento Peninsula (Italy), which isthought to play a crucial role as a crossroads between different biogeographic areas. Analysis of biogeography,the assignment of species into 6 bioclimatic categories, a novel method to assess intermatrixcorrelation significance and the correlation between species distribution, and some environmentalvariables provided relevant tools to investigate the influence of a suite of ecological andhistorical factors on syllid distribution. Data showed that Syllidae could be considered a useful taxonfor biogeographic speculations, even though bioclimatic and environmental analyses appeared significantlymore informative. The Salento Peninsula revealed an affinity with some eastern Mediterraneancoastlines. Syllid distribution suggested the existence of an ‘Atlantism’ gradient, decreasingeastwards and possibly corresponding to a temperature gradient when bioclimatic categories wereanalysed. In contrast to previous results, the Eastern and Western Basins did not show great differencesin species diversity. The observed differences could, in part, be due to an ‘author effect’, due tothe differing taxonomic updating of the available faunistic lists. Syllidae were found to be effectivefaunistic and ecological indicators, able to characterize different areas inside the Mediterranean Sea;thus, the present results could be used to stimulate further research on different aspects of the family.