Precise descriptions and comprehensive taxonomies of species and their ecology are essential in monitoring changes in marine biodiversity at multiple spatial scales. A currently undescribed species of commensal amphipod in the genus Leucothoe is reported from New Zealand, collected from the endemic tunicate Cnemidocarpa bicornuta. This species differs from others in the genus in having a one-articulate first maxilla palp and an apically produced tuberculate lobe on the inner margin of the outer plate of the maxilliped. Previous taxonomic surveys in New Zealand waters did not document this species, indicating that it may be a recent arrival. This research highlights the importance of biodiversity monitoring and taxonomic surveys to record occurrences of undescribed or recently-arrived taxa.