Aims: Recent studies have demonstrated that microRNAs (miRNAs) can serve as useful biomarkers for human cancers. The aim of this study was to evaluate the expression level of serum miRNA-206 in patients with gastric cancer (GC) and investigate its diagnostic and prognostic value. Methods: Quantitative real-time PCR was performed to evaluate serum miRNA-206 levels in 150 GC patients and 150 healthy volunteers. The association between miRNA-206 expression and clinicopathological factors as well as patient's survival was analyzed. Receiver operator curve (ROC) analysis was carried out to assess the potential value of serum miRNA-206 for GC diagnosis. Results: Serum miRNA-206 was down-regulated in GC patients compared with healthy controls (P < 0.001). Decreased serum miRNA-206 expression was significantly associated with deep local invasion, positive lymph node metastasis, and advanced clinical stage. Serum miRNA-206 expression was found to be significantly up-regulated in paired post-operative samples and reduced in patients with GC recurrence. ROC curve analysis showed that serum miRNA-206 was a useful marker for GC diagnosis, and could discriminate between recurred and non-recurred patients. Multivariate Cox regression analysis confirmed low serum miRNA-206 expression as an independent unfavorable prognostic factor for both DFS and OS of GC patients. Conclusions: These results suggest that serum miRNA-206 might not only serve as a novel diagnostic biomarker for GC, but also predict cancer recurrence and patient's prognosis.