Frailty is a multifactorial geriatric condition linked to increased mortality and adverse health outcomes and is associated with gut microbiome features that differ from those observed in healthy ageing. We analyze gut metagenomic profiles in relation to estimated frailty severity and frailty-related clinical outcomes assessed with an internally developed and validated Frailty Mortality Index (FMI) in the SUPERB cohort, comprising 2,081 Swedish women aged 75–80 years. The FMI is a composite measure that integrates functional, physiological and psychological dimensions associated with frailty and mortality risk, and shows stronger associations with mortality compared to the Charlson Comorbidity Index in the SUPERB cohort. The FMI is inversely associated with microbial diversity, gene richness, and predicted functional capacity, which are linked to physical function, mortality and fall-related injuries. A total of 404 bacterial species are significantly associated with FMI, and most show concordant associations in a Chinese cohort of 1,448 older adults. Here we show microbial signatures linked to frailty and mortality across different continents.