Objective To investigate the effects of metformin on proliferation and autophagy in gastric cancer cells. Methods Human gastric cancer cell lines AGS and HGC-27 were treated with metformin in vitro, and the cell proliferation, apoptosis and protein expression of cysteine aspartic acid specific protease 3 (caspase 3) and microtubule-associated protein 1 light chain 3 (LC3) were detected by cell counting kit 8 assay, flow cytometry and Western blotting, respectively. In vivo, a nude mouse model of subcutaneous xenograft tumor was established using AGS cells. When tumor volume reached approximately 100 mm³, mice were randomly assigned to 4 groups (n＝8): control group, chloroquine group, metformin group, or metformin combined with chloroquine group. Mice were intragastrically administered normal saline, chloroquine (50 mg/kg), metformin (100 mg/kg), and metformin plus chloroquine daily for 3 consecutive weeks, respectively. Tumor volume and weight were measured and recorded regularly during the treatment. Results Metformin inhibited the proliferation of AGS and HGC-27 cells with half inhibitory concentrations of 5.72 and 13.32 mmol/L, respectively, induced apoptosis accompanied by caspase 3 activation, and triggered autophagy indicated by elevated LC3-Ⅱ expression (all P＜0.01). Combined treatment with metformin and chloroquine further reduced cell viability (P＜0.01). In vivo experiments showed that tumor volume and weight were significantly lower in the metformin group than those in the control group (both P＜0.01), and further reduction was observed following combined treatment with chloroquine (both P＜0.01). Conclusion Metformin suppresses gastric cancer growth by inhibiting cell proliferation, inducing apoptosis, and activating protective autophagy, and combined treatment with the autophagy inhibitor chloroquine further enhances its antitumor efficacy in vitro and in vivo.