<p>As the global prevalence of diabetes continues to rise, an increasing number of fracture patients present with metabolic comorbidities, raising concerns regarding their healing outcomes. This systematic review and meta-analysis aimed to provide an updated and comprehensive assessment of the association between diabetes and delayed union or non-union. A comprehensive search of PubMed, Embase, Web of Science, Scopus, CINAHL, and the Cochrane Library was performed from inception to November 2025, following PRISMA guidelines. Two reviewers independently performed study selection, data extraction, and quality assessment. Odds ratios (ORs) with 95% confidence intervals (CIs) were pooled using random-effects models. Heterogeneity was quantified using the I² statistic, and subgroup and sensitivity analyses were conducted to explore potential effect modifiers. Study quality was evaluated using the Newcastle–Ottawa Scale (NOS), and publication bias was assessed using funnel plots, Begg’s test, and Egger’s test. Overall, diabetes was associated with an increased risk of delayed union or non-union (OR = 1.61, 95% CI 1.44–1.81; I² = 76.6%). Sensitivity analysis restricted to high-quality studies (n = 30) yielded consistent results (OR = 1.60, 95% CI 1.56–1.65), indicating robust findings. Subgroup analyses suggested that the magnitude of association varied by fracture location, diabetes type, and diabetes severity. Type 1 diabetes, complicated diabetes, and diaphyseal long-bone fractures demonstrated particularly elevated risks. No significant publication bias was detected. In conclusion, diabetes is associated with an increased risk of delayed union or non-union, and this effect appears more pronounced in certain clinical subgroups. These findings highlight the importance of considering diabetes status when assessing fracture-healing risk and emphasize the need for future prospective studies with standardized reporting of glycemic control and diabetes severity to refine risk stratification.</p>