Phidiana hiltoni is a conspicuous nudibranch sea slug native to the northeastern Pacific Ocean. Over the past thirty years the range of P. hiltoni has expanded about 200 km northward, but the mechanism that facilitated this expansion is poorly understood. In this study, we use mtDNA and microsatellite data to investigate the population structure of P. hiltoni in its historical range as well as in recently colonized localities. Microsatellite analyses reveal little to no genetic structure and thus high gene flow throughout the range of P. hiltoni. This is consistent with mtDNA analysis results, which revealed shared haplotypes between Southern, Central and Northern populations. However, AMOVA of mtDNA data did recover some genetic structure among geographic regions. This, along with same group memberships in the microsatellite data of individuals from sites like Cave Landing, suggest a certain degree of local recruitment and reduced vagility. Recently established populations in Northern California contain two unique mtDNA haplotypes that are not present elsewhere, but microsatellite data do not differentiate these from other populations. The mismatch between mtDNA and microsatellite data could be explained by the mating system of this aggressive, hermaphroditic species as well as the sporadic nature of the northward dispersal. Analyses of historical abundance data of P. hiltoni suggest a population decline in Southern California. Together, these results suggest a northward population shift, rather than a range expansion, possibly related to ongoing changes in nearshore oceanographic conditions in the region.