Background <p>Chronic diarrhea and constipation are known to be associated with obesity. Body roundness index (BRI), as a novel physical dimension assessment indicator, provides a more comprehensive evaluation of body and visceral fat than traditional methods. However, the relationship between BRI, chronic diarrhea, and constipation remains unclear. We aimed to investigate the relationship between BRI, chronic diarrhea, and constipation.</p> Methods <p>A cross-sectional study based on the National Health and Nutrition Examination Survey 2005–2010 was conducted. Weighted multivariable logistic regression was used to analyze the association between BRI, chronic diarrhea, and constipation. Restricted cubic spline curves were plotted to verify the linear associations.</p> Results <p>7182 participants were included in this study, among whom 491 had chronic diarrhea and 441 had constipation. Significant positive correlations were discovered between BRI and chronic diarrhea, while no correlation was detected with constipation in the fully adjusted multivariable logistic regression analysis. Restricted cubic spline curves confirmed the linear relationship described above. Further treating BRI as categorical variables, compared with the lowest tertile, the highest BRI tertile showed a 79% increase in chronic diarrhea incidence and a 35% decrease in chronic constipation incidence. Consistent findings were observed across different subgroups, and sensitivity analyses generally confirmed the robustness of our results.</p> Conclusions <p>BRI is significantly and linearly associated with chronic diarrhea. Higher body and visceral fat increase the risk of chronic diarrhea while reducing the risk of chronic constipation.</p>