Invasive and non-native species can pose risks to vulnerable ecosystems by co-introducing bacterial pathogens. Alternatively, co-introduced bacterial pathogens may regulate invasive population size and invasive traits. We describe a novel candidate genus and species of bacteria (‘Candidatus Aquirickettsiella gammari’) found to infect Gammarus fossarum, from its native range in Poland. The bacterium develops intracellularly within the haemocytes and cells of the musculature, hepatopancreas, connective tissues, nervous system and gonad of the host. The developmental cycle of ‘Candidatus Aquirickettsiella gammari’ includes an elementary body (496.73 nm ± 37.56 nm in length, and 176.89 nm ± 36.29 nm in width), an elliptical, condensed spherical stage (737.61 nm ± 44.51 nm in length and 300.07 nm ± 44.02 nm in width), a divisional stage, and a spherical initial body (1397.59 nm ± 21.26 nm in diameter). We provide a partial genome for ‘Candidatus Aquirickettsiella gammari’ which clades phylogenetically alongside environmental 16S rRNA sequences from aquatic habitats, and bacterial symbionts from aquatic isopods (Asellus aquaticus), grouping separately from the Rickettsiella, a genus that includes bacterial pathogens of terrestrial insects and isopods. Increased understanding of the diversity of symbionts carried by G. fossarum identifies those that might regulate host population size, or those that could pose a risk to native species in the invasive range. Identification of ‘Candidatus Aquirickettsiella gammari’ and its potential for adaptation as a biological control agent is explored.