The shell-boring Polydora websteri is described in detail in the present study for future unambiguous identification using an integrative taxonomic approach that combines morphology and molecular analysis of the mitochondrial cytochrome c oxidase subunit 1 (COI) gene. Adult P. websteri exhibit a high degree of morphological plasticity in the palp pigmentation pattern, the shape of the anterior edge of the prostomium, the shape of the major spines on chaetiger 5, and the shape of the pygidium. The COI gene sequence demonstrated that the intraspecific distance of P. websteri was 0.33%, whereas the interspecific distance of P. websteri ranged from 18.88% (with P. brevipalpa) to 24.79% (with Boccardia proboscidea). The intraspecific genetic distances of polydorids examined in the present study ranged from 0.33% to 1.67%, whereas the interspecific distances ranged from 18.88% to 24.79%. Such large barcoding gaps between intra- and inter-specific distances indicate that the COI is a suitable gene marker for molecular identification of polydorid species. Our results demonstrate that not only did all COI sequences from the larvae show greater than 99% sequence identity to those from adults, but some larvae share the same haplotypes as adults. These findings clearly indicate that the larvae collected from sea waters around an oyster farm belong to P. websteri, the same species as the adult worms collected from the oyster Crassostrea hongkongensis in that locality. Two polydorid-specific primers were successfully designed, for the first time, to amplify target fragments of the COI gene. This study is the first to molecularly validate unidentified larvae from the aquatic environment through the known COI sequences of adults.