Circulating microRNAs (miRNAs) exhibit altered expression in patients with cancer and could be considered as potential prognostic biomarker of cancer. Here, we performed a meta-analysis to summarize all the results from available studies, aiming to analyze the prognostic role of circulating microRNA-21 (miR-21) in human cancers. Eligible studies were identified from PubMed and EMBASE through multiple search strategies. We extracted and estimated the hazard ratios (HRs) for overall survival (OS), which compared the high and low expression levels of circulating miR-21 in patients with a variety of carcinomas. Pooled HRs and 95 % confidence intervals (CIs) were calculated. Eleven studies with a total of 1,224 patients with various carcinomas were included this meta-analysis. For OS, higher circulating miR-21expression could significantly predict worse outcome with the pooled HR of 2.11 (95 % CI 1.36-3.26, P = 0.0009). The subgroup analysis suggested that the elevated circulating miR-21 expression was correlated with worse OS in Asian population with the pooled HR of 2.36 (95 % CI 1.61-3.48, P < 0.0001) and digestive system cancers with the pooled HR of 2.19 (95 % CI 1.01-4.75, P = 0.05). The present meta-analysis suggests that circulating miR-21 expression is associated with poor survival in patients with cancer and could be a prognostic biomarker for those patients.