MicroRNA-138 (miR-138) has been reported to be downregulated and function as a tumor suppressor in several cancers. However, the role and molecular mechanisms of miR-138 in the progression of gastric cancer (GC) remain to be clarified. The aim of the present study was to determine the role of miR-138 in GC progression. In the present study we found that miR-138 expression was downregulated in GC tissues and cell lines. Statistical analysis demonstrated that low expression levels of miR-138 were associated with advanced tumor-node-metastasis (TNM) stage, and lymph node metastasis. Function assays demonstrated that overexpression of miR-138 impaired GC cell proliferation, colony formation, migration and invasion in vitro, as well as suppressed tumor growth in vivo. Through reporter gene, qRT-PCR and western blot assays, SRY-related high mobility group box 4 (SOX4), a master mediator in epithelial-mesenchymal transition (EMT), was confirmed to be a direct target of miR-138 in GC cells. Western blot assay revealed that miR-138 overexpression inhibited EMT procession in GC cells by upregulation of epithelial marker E-cadherin and downregulation of mesenchymal markers, N-cadherin and vimentin. Furthermore, the levels of miR-138 were inversely correlated with those of SOX4 expression in GC tissues. Overexpression of SOX4 rescued the inhibition effect in GC cells caused by miR-138. Collectively, these findings indicate that miR-138 may be a potential therapeutic target for GC.