Three species of Oncaeidae, Triconia umerus, T. hawii and T. rufa are reported for the first time in the coastal waters of the Gulf of Naples, Tyrrhenian Sea, western Mediterranean Sea. They were found in meso-zooplankton samples collected in 2004 and were absent from samples collected prior to 2004. Specimens of Triconia umerus and T. hawii were compared with material collected from the type locality (the Red Sea). In order to facilitate identification, brief differential diagnoses, supported by scanning electron micrographs, are presented for the five species of Triconia that have small body size (450 to 600 μm) and co-occur in the upper 50 m of the water column of the Gulf of Naples. Surface ornamentation on the genital double-somite of the female is reported in T. minuta and T. hawii for the first time, but it is difficult to observe using light microscopy and we infer that it has probably been overlooked in these species hitherto. Possible explanations for the new discovery of these species in such a well-studied area are discussed and it is suggested that they represent relatively recent additions to the fauna. A key to the eleven species of Oncaeidae found in the Gulf of Naples is presented.