Early-onset colorectal cancer (EoCRC) constitutes 2%–10% of all colorectal cancers and is becoming more common globally. Diabetes mellitus (DM) has increased substantially in younger adults; however, its involvement in EoCRC remains unclear. We conducted a systematic review and meta-analysis of observational studies to (1) explore the prevalence of DM in individuals with EoCRC and (2) investigate the association between DM and EoCRC. A systematic literature search was conducted to identify studies published before May 2022 that evaluated the association between DM and EoCRC risk in four databases, including Medline (PubMed), Embase, Web of Science, and the Cochrane Central Register of Controlled Trials. Results from the studies were summarized in meta-analyses using random effects models. Nineteen eligible studies were included. A total of 33,359 EoCRC cases and 14,259,289 controls in 12 studies were included in the meta-analysis. The pooled odds ratio OR of 1.43 (95% confidence interval CI: 1.08–1.8) indicated significant positive association between DM and increased risk of EoCRC. Subgroup analysis demonstrated that diabetes severity was significantly associated with unmanaged DM (OR = 1.28, 95% CI: 1.02–1.6), but not with managed DM (OR = 1.04, 95% CI: 0.84–1.28). Our results suggest that DM is a risk factor for EoCRC, and the higher prevalence of DM among younger adults may contribute to the increasing incidence of EoCRC. Interventions to reduce this bidirectional risk should be further investigated for the development of effective prevention and treatment strategies. PROSPERO registration: CRD42022306347. •This study collected large sample size using a multicenter, multinational, multiracial, and multicontinental approach.•The rising prevalence of DM among younger adults may partially contribute to the increasing incidence of EoCRC.•Active screening for diabetes should be considered in EoCRC patients.