The study of hard-bottom Syllidae (Polychaeta) of the Rovinj area provides an updated measure of syllid species diversity in the northern Adriatic Sea. Faunal research in the coldest Mediterranean sectors may also help to address possible ecological and biogeographical consequences arising from climate change. Thirty-nine species were found, of which 13 are new for the northern Adriatic, increasing the species recorded from the area from 53 to 66. Some newly recorded species are dominant and typical of warmer areas. The lack of previous taxonomic updates is responsible for the high number of new findings. However, the northern Adriatic Sea is possibly undergoing long-term changes, with modifications of diversity due to the establishment of warm-water species. Syllid fauna of Sveti Ivan Island and its bioclimatic affinity are, in fact, unexpectedly more similar to the ones of some southern Mediterranean areas than to those traditionally reported for the northern Adriatic.