Objective <p>Endometrial cancer is one of the most common gynecological malignancies with an incidence rising globally in parallel with the increasing prevalence of diabetes mellitus. The objective of the present study was to provide an updated and comprehensive synthesis of the evidence on the association between diabetes and the risk of endometrial cancer, while accounting for study design, methodological quality plus population characteristics.</p> Methods <p>A systematic review and meta-analysis was undertaken in accordance with PRISMA 2020 guidelines. PubMed/MEDLINE, Embase, Web of Science, Scopus and Cochrane Library were searched from inception to 15 Aug 2025 alongside; eligible designs were observational studies assessing diabetes mellitus and incident endometrial cancer. Risk of bias was assessed using the Newcastle–Ottawa Scale and ROBINS-I. The most fully adjusted estimate from each study was extracted, transformed to the log-relative risk scale and pooled using a random-effects model with restricted maximum likelihood estimation. Lastly, prespecified subgroup analyses examined differences by geographic region, menopausal status, diabetes type, study base, study design plus study quality.</p> Results <p>Thirty-five studies met the inclusion criteria and contributed 37 risk estimates to the quantitative synthesis. Diabetes mellitus was found to be associated with a statistically significant higher risk of endometrial cancer incidence; resulting in a pooled relative risk of 1.64 (95% CI 1.50–1.80). Substantial heterogeneity was observed but was partly explained by study base and quality; as hospital-based and lower-quality studies reported stronger associations rather than population-based and higher-quality cohorts. The found association was consistent across continents, menopausal groups and diabetes types, including gestational diabetes. Ultimately, funnel plot asymmetry and results from Egger’s test suggested small-study effects, although sensitivity analyses, leave-one-out and cumulative meta-analysis, demonstrated that the overall association remained robust.</p> Conclusion <p>Diabetes mellitus is associated with a materially increased risk of endometrial cancer, independent of adiposity. Moreover, this relationship has been consistently demonstrated across diverse populations and study designs. These findings underscore the importance of targeted preventive strategies in women with diabetes mellitus and highlight the need for further research addressing diabetes duration, glycemic control alongside treatment effects on cancer risk.</p> Trial registration <p>PROSPERO CRD420251142624.</p>