Purpose: Diabetes mellitus, a frequent comorbid condition in cancer patients, has been shown to increase risk of all-site cancer mortality. This relationship has not been systematically studied in bladder cancer patients. This systematic review and meta-analysis aimed to identify, evaluate, and synthesize available evidence on the relationship between history of diabetes and outcomes in bladder cancer patients. Materials and Methods: Systematic searches interrogated OVID MEDLINE, Embase, Web of Science, Google Scholar, and Cochrane Library to identify scholarly reports relating diabetes to all-cause mortality, bladder cancer-specific mortality, recurrence, and progression in bladder cancer patients. After critical review, meta-analysis was used to quantitively synthesize qualifying data and assess potential influence of publication bias, clinical heterogeneity, and residual confounding. Results: We synthesized data on over 226,472 patients treated with curative intent uniquely represented in 28 studies that met quality metrics. Having diabetes was positively associated with each outcome. Hazard ratio estimates were indistinguishable for mortality from any cause, 1.22 (95% confidence interval [CI], 1.12–1.33) and bladder cancer-specific mortality, 1.28 (95% CI 1.17–1.41) and notably stronger in patients with muscle-invasive and high-risk non–muscle-invasive bladder cancer, 1.32 (95% CI, 1.15–1.50) and 1.48 (95% CI, 1.06–2.06). Neither publication bias, systematic error, nor confounding by factors such as smoking or obesity is likely to explain the observed associations. Conclusions Bladder cancer patients with diabetes experience elevated mortality that is not explained by diabetes-related comorbidities or complications. Future research should explore type, severity, and duration of diabetes in relation to unfavorable bladder cancer outcomes.