We explored temporal patterns of genetic diversity and spatial genetic structure of the recently introduced ascidian Perophora japonica Oka, 1927 in Europe. A fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI) was sequenced for 291 colonies of one population in Plymouth, UK, which was monitored for 9 yr after its initial discovery. A total of 238 colonies from 12 localities were also sequenced for population structure analyses. The temporal monitoring of the Plymouth population showed a progressive loss of genetic diversity over time attributable to a strong initial bottleneck followed by genetic drift and/or selection. Population genetic structure was consistent with the historical records of this introduction, which probably originated from oyster farming activities in France, from where the species spread to the UK and Spain. Only one population in France displayed high levels of genetic diversity, and most of the remaining populations presented very low variability. In addition, significant differentiation in terms of allele frequencies was detected between some populations. P. japonica has suffered a loss of genetic diversity in both space and time since its introduction, but this did not prevent its expansion. Accidental human transport is the most likely mechanism of spread within the introduced range. Asexual propagation modes and chimerism in this species may play an important role in introduction events. The genetic data presented here can contribute to the design of more efficient management methods for this and similar introduced-species.