The aim of this manuscript is to analyze the diagnostic and prognostic value of circulating miR-18a in the plasma of patients with gastric cancer. In this study, 82 patients with gastric cancer and 65 healthy controls were enrolled in the study, and 10 ml of peripheral venous blood was collected for RNA extraction. miR-18a expression was determined using TaqMan quantitative real-time polymerase chain reaction assay and was further correlated with patients’ clinicopathological parameters and the follow-up data. The results indicated that plasma miR-18a was upregulated in gastric cancer patients compared with healthy controls (P < 0.001). miR-18a yielded an area under the receiver-operating characteristic curve of 0.907 with 80.5 % sensitivity and 84.6 % specificity in discriminating gastric cancer from healthy controls. Plasma miR-18a expression was significantly associated with pathological grade (P = 0.036) and lymph node status (P = 0.025), but not with tumor stage (P = 0.075). Both log-rank test and univariate Cox regression analysis showed that the higher miR-18a expression in plasma was associated with shorter disease-free survival and disease-specific survival of the patients with gastric cancer (P = 0.023 and P = 0.027; P = 0.036 and P = 0.043, respectively), which was also not proven by multivariate Cox regression analysis (P = 0.238 and P = 0.160, respectively). In conclusion, this study showed that miR-18a may be a promising biomarker for the detection of gastric cancer and its upregulation may be potentially associated with unfavorable prognosis of bladder cancer, suggesting that miR-18a might serve as a potential biological marker for further risk stratification in the management of gastric cancer.