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• Protective autophagy induced by DDP decreases chemosensitivity in GC cells. • MGMT inhibited ATG4B to repress autophagy. • Inhibition of autophagy enhanced the chemosensitivity to DDP in GC xenograft model. • High MGMT expression and low ATG4B expression are significantly correlated with survival of GC. Cisplatin (DDP) is the first-line drug for the treatment of gastric cancer (GC). However, DDP resistance is common. Autophagy, which is closely related to chemoresistance, is a process of resolving and recycling proteins and damaged cellular organs. Additionally, O-6-methylguanine-DNA methyltransferase (MGMT) is responsible for alkylating drug resistance. However, the relationship between autophagy and MGMT in response to DDP in GC is still unknown. In the present study, we determined that autophagy induced by DDP decreases chemosensitivity in GC cell lines. DDP may have induced autophagy in GC by inhibiting MGMT to increase autophagy-related gene (ATG) 4B. Inhibition of MGMT-mediated ATG4B suppression resulted in autophagy induction and DDP resistance. In vivo , combined DDP and autophagy inhibitor chloroquine (CQ) enhanced the anti-tumor effect of DDP; additionally, the negative correlation of MGMT and ATG4B was confirmed. High expression of MGMT and low expression of ATG4B were significantly correlated with favorable five-year survival rate (P < 0.05) in 66 clinicopathologically characterized GC cases. Our study demonstrate that DDP inhibits MGMT-mediated autophagy suppression to decrease chemosensitivity in GC, which provides a novel therapeutic strategy to promote DDP chemosensitivity in GC. [ABSTRACT FROM AUTHOR]