The prevalence of diabetes mellitus (DM) and acute pancreatitis (AP) increases continuously, therefore, to understand the effects of preexisting diabetes on AP is crucially needed. Here, we performed a systematic review and meta-analysis in which AP patients including DM and non-DM groups were sorted. Several outcome parameters were analyzed, and the odds ratio (OR) and standardized mean difference with 95% confidence intervals (CIs) were calculated.We found 1417 articles, of which 9 articles involving 354,880 patients were analyzed. More complications were seen in diabetic patients than in non-DM patients (OR, 1.553 [95% CI, 1.266-1.904]; P < 0.001). Intensive care unit admission (OR, 1.799 [95% CI, 1.442-2.243]; P < 0.001) and renal failure (OR, 1.585 [95% CI, 1.278-1.966]; P < 0.001) were more frequent in DM patients. There was a tendency of higher mortality and local complications (OR, 1.276 [95% CI, 0.991-1.643]; P = 0.059; and OR, 1.267 [95% CI, 0.964-1.659]; P = 0.090, respectively) in preexisting DM. Length of hospitalization was longer in DM patients (standardized mean difference, 0.217 [95% CI, 0.075-0.360]; P = 0.003). Preexisting DM negatively influences the outcome of AP and increases the risk of renal failure, local complications, and mortality.