Background/aim: A disintegrin and metalloprotease (ADAM) 12 expression has been found up-regulated in various cancer types. The aim of the study was to evaluate whether ADAM12 affects oncogenic behavior of gastric cancer (GC) cells and investigate its prognostic value. Materials and methods: The effect of ADAM12 on tumor cell behavior was examined using the small interfering RNA and pcDNA6-myc vector in human GC cell lines. Expression of ADAM12 in GC tissues was confirmed by immunohistochemistry. Apoptosis and proliferation were determined by a terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay and immunohistochemical staining for Ki-67. Results: ADAM12 overexpression enhanced tumor cell migration and invasion in AGS and SNU638 cells. Down-regulation of caspase-3 and PARP activity due to ADAM12 overexpression enhanced tumor cell proliferation and inhibited apoptosis. The expression of Snail and Vimentin increased and that of E-cadherin decreased following ADAM12 overexpression. In contrast, ADAM12 knockdown reversed these effects. ADAM12 overexpression increased the phosphorylation of Akt and GSK-3β. The mean Ki-67 labeling index value of ADAM12-positive tumors was significantly higher compared to that of ADAM12-negative tumors. ADAM12 expression was associated with age, tumor size, cancer stage, depth of invasion, lymph node metastasis, and poor survival. Conclusion: ADAM12 enhances tumor progression by increasing cell mobility, enhancing cell proliferation, and inhibiting apoptosis in GC cells. Also, ADAM12 is associated with adverse clinicopathological features and poor survival. It may be used as a molecular marker for the prediction of clinical outcomes of patients with GC.Keywords: ADAM12; cell survival; neoplasm; prognosis; stomach.