Polychaete worms are known to thrive in extreme environmental conditions, however little is known about how polychaete species will respond to major climatic stressors, such as ocean acidification. Here, we examined the distribution of Fabriciidae (Annelida, Sabellida) species along a gradient of ocean acidification, caused by carbon dioxide (CO2) vent emissions in a shallow, coastal system off the island of Ischia (Tyrrhenian Sea, Italy). A total of 265 specimens of Fabriciidae, representing six species from five genera, were collected along the gradient. Most of the species were found across the entire CO2 gradient, suggesting polychaetes may have a high tolerance for ocean acidification in the future. Two of the species were new to science, and two of the genera were previously unrecorded in the Mediterranean. A full description of the new species Brifacia aragonensis sp. nov. and Parafabricia mazzellae sp. nov. is given, both of which were most abundant in the most acidified areas (pH 6.6–7.2). The geographical distribution and ecology of the new taxa, as well as of the other fabriciid species collected, is discussed. Taxonomic keys to identify the Fabriciidae species currently recorded in the Mediterranean Sea are also provided.