The molecular mechanism underlying microRNA (miR)-17 overexpression has not been clearly evaluated in gastric cancer. We aimed to evaluate the functional roles of miR-17 in gastric cancer and test its viability as a therapeutic target. We conducted comparative genomic hybridization and expression array analyses on human gastric cancer tissue samples, as well as evaluating the functional roles of miR-17 in gastric cancer cell lines and transgenic mice. miR-17 overexpression in gastric cancer patients was associated with copy number gain of proliferation-associated oncogenes such asMYC,CCNE1,ERBB2, andFGFR2. Copy number gain ofMIR17HGgene (13q31.3) was rare, with an overall frequency of 2% in gastric cancers (1 of 51). miR-17 knockdown suppressed the monolayer and anchorage-independent growth ofFGFR2-amplified KATO-III gastric cancer cells.mir-17–92 TG/TGmice overexpressing themir-17–92cluster under the villin promoter developed spontaneous benign tumors in the intestinal tract (log-rankPfor tumor-free survival = 0.069). Taken together, miR-17 overexpression in gastric cancer was rarely associated withMIR17HGgene amplification, but correlated with proliferation-associated oncogene amplification. Therefore, miR-17-targeting approach may benefit patients with gastric cancers harboring proliferation-associated oncogene amplification.