Resistance to cisplatin (DDP) remains a major obstacle in the control of gastric cancer (GC) progression. A previous study revealed that microRNA-21 (miR-21) contributes to DDP resistance in GC cells via the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathway. The aim of the current study was to explore the mechanisms underlying the cytoprotective function of miR-21. In this study, DDP-resistant GC cells were obtained by continuous exposure of human gastric adenocarcinoma cells to increasing concentrations of DDP. Western blot analysis was used to evaluate activation of the PI3K/Akt/mechanistic target of rapamycin kinase (mTOR) pathway. The level of miR-21 was altered by transfection of miR-21 mimic and inhibitor. Autophagy was assessed by detecting autophagosome formation, Beclin-1 and LC3 expression. An Annexin V-propidium iodide assay was performed to estimate the survival and death of GC cells. GC cells became refractory to the growth inhibition and apoptosis induced by DDP treatment, activation of Akt and mTOR were increased in DDP-resistant GC cells. Inhibition of autophagy decreased the sensitivity of GC cells to DDP, and autophagy induction produced the opposite effect. DDP-resistant GC cells expressed higher levels of miR-21 compared with the parent cells. Transfection of GC cells with miR-21 mimics contributed to restored DDP resistance by suppressing autophagy, while miR-21 inhibitor sensitized DDP-resistant GC cells by promoting autophagy. In conclusion, the results demonstrated that miR-21 is associated with DDP resistance in GC cells by inhibiting autophagy via the PI3K/Akt/mTOR pathway, and autophagy inducers could be therapeutic targets for the effective treatment of DDP resistance in GC.