To quantitatively evaluate the effect of preexisting diabetes mellitus (DM) on the outcomes of patients with non-small-cell lung cancer (NSCLC).Observational studies comparing the prognosis of NSCLC patients with and without diabetes were identified from PubMed, EMBASE, and The Cochrane Central Register of Controlled Trials (CENTRAL). We searched for studies that published in English from inception to March 30, 2019, using search terms related to diabetes and NSCLC. Pooled hazard ratio (HR) and 95% confidence interval (CI) were calculated by a random-effect model and subgroup analyses were performed.In all, 17 of 1475 identified studies were finally included in the meta-analysis. The result revealed that preexisting diabetes had a significantly negative impact on the overall survival (OS) of patients with NSCLC (HR: 1.31, 95% CI: 1.12-1.54), especially in patients undergoing surgical treatment (HR: 1.46, 95% CI: 1.02-2.09) in comparison with those receiving only non-surgical treatment (HR: 1.33, 95% CI: 0.87-2.03). In addition, preexisting diabetes was more likely to be associated with a worse prognosis among Asian NSCLC patients than Western patients. Sensitivity analysis indicated that the main result was robust, and no evidence of publication bias was found.Preexisting DM has a negative impact on diabetic NSCLC patients' prognosis.