ABSTRACTRecent evidence emphasized the role of the microbiota in the etiopathogenesis of rheumatoid arthritis (RA). Indeed, it has been demonstrated that urinary tract infections are implicated in RA pathogenesis. However, a definitive association between the urinary tract microbiota and RA remains to be investigated. Urine samples from 39 patients affected by RA, including treatment-naive patients, and 37 age- and sex-matched healthy individuals were collected. In RA patients, the urinary microbiota showed an increase in microbial richness and a decrease in microbial dissimilarity, especially in treatment-naive patients. A total of 48 altered genera with different absolute quantities were detected in patients with RA. The 37 enriched genera includedProteus,Faecalibacterium, andBacteroides, while the 11 deficient genera includedGardnerella,Ruminococcus,Megasphaera, andUreaplasma. Notably, the more abundant genera in RA patients were correlated with the disease activity score of 28 joints-erythrocyte sedimentation rates (DAS28-ESR) and an increase in plasma B cells. Furthermore, the altered urinary metabolites, such as proline, citric acid, and oxalic acid, were positively associated with RA patients, and they were closely correlated with urinary microbiota. These findings suggested a strong association between the altered urinary microbiota and metabolites with disease severity and dysregulated immune responses in RA patients.IMPORTANCEWe revealed that the profile of the urinary tract microbiota in RA featured with increased microbial richness and shifted taxa, associated with immunological and metabolic changes of the disease, underlining the interplay between urinary microbiota and host autoimmunity.