Rubyspira osteovora is an unusual deep-sea snail from Monterey Canyon, California. This group has only been found on decomposing whales and is thought to use bone as a novel source of nutrition. This study characterized the gut microbiome of R. osteovora, compared to the surrounding environment, as well as to other deep-sea snails with more typical diets. Analysis of 16S rRNA gene sequences revealed that R. osteovora digestive tissues host a much lower bacterial diversity (average Shannon index of 1.9; n = 12), compared to environmental samples (average Shannon index of 4.4; n = 2) and are dominated by two bacterial genera: Mycoplasma and Psychromonas (comprising up to 56% and 42% average total recovered sequences, respectively). These two bacteria, along with Psychrilyobacter sp. (∼16% average recovered sequences), accounted for between 43% and 92% of the total recovered sequences in individual snail digestive systems, with other OTUs present at much lower proportions. The relative abundance of these three groups remained similar over 6 years of sampling (collection date was not shown to be a significant predictor of community structure), suggesting a long-term association. Furthermore, these bacterial genera were either not present (Mycoplasma and Psychromonas) or at very low abundance (<0.04% for Psychrilyobacter), in environmental samples and other deep-sea gastropods, supporting the uniqueness of the R. osteovora gut microbiome.