Cryptic diversity, i.e. diversity observable in genetic but not in morphological traits, is prevalent in marine invertebrates and presents one of the greatest obstacles to obtaining accurate estimates of species richness. Sipunculans, commonly called peanut worms, are marine annelids in which high levels of cryptic diversity have previously been documented. In this study, we use genetic identification techniques to examine divergence of two lineages of Pacific sipunculans, both known under the namePhascolosoma agassizii. One lineage is isolated to the eastern Pacific coast while the other one inhabits the western Pacific coast. These clades are reciprocally monophyletic and are not recovered as sister taxa, suggesting relatively early divergence withinPhascolosoma. Furthermore, we did not find support for a genetic distinction between the western PacificPhascolosoma agassizii agassiziiandPhascolosoma agassizii kurilense, a subspecies reported from the Kuril Islands. Considering that the type locality forP. agassiziiis in the Eastern Pacific, we suggest that the western clade, including the samples from the Kuril islands, represent a new, undescribed species.