The human gut microbiome is critical to health and wellbeing. It hosts a complex ecosystem comprising a multitude of bacterial species, which contributes functionality that would otherwise be absent from the host. Transient and commensal bacteria in the gut must withstand many stresses. The influence of mobile genetic elements such as plasmids in stress adaptation within the ecosystem is poorly understood. Using a mobilomic approach we found evidence for plasmid-mediated osmotolerance as a phenotype amongst the Proteobacteria in healthy faecal slurries. A transconjugant carrying multiple plasmids acquired from healthy faecal slurry demonstrated increased osmotolerance in the presence of metal salts, particularly potassium chloride, which was not evident in the recipient. Pyrosequencing and analysis of the total plasmid DNA demonstrated the presence of plasmid-borne osmotolerance systems (including KdpD and H-NS) which may be linked to the observed phenotype. This is the first report of a transferable osmotolerance phenotype in gut commensals and may have implications for the transfer of osmotolerance in other niches.