Cisplatin-based chemotherapy resistance is a common issue for cancer clinical efficacy. Metformin is being studied for its possible anticancer effect. The present study aimed to investigate whether metformin affects the chemosensitivity of gastric cancer to cisplatin and reveal the molecular mechanism. In this study, the effects of combination therapy with metformin and cisplatin on cell viability, cell apoptosis, malondialdehyde, superoxide dismutase, reactive oxygen species level, glucose uptake, lactate production, protein level, and xenograft tumor formation were analyzed in gastric cancer cells. Immunohistochemical staining was performed to detect Ki67 expression in matched tumor samples. The results showed that NCI-N87 and SNU-16 cells were most resistant and sensitive to cisplatin, respectively. Metformin treatment increased the cisplatin sensitivity of gastric cancer by inhibiting cell viability and metabolic reprogramming and promoting cell apoptosis and oxidative stress. Furthermore, overexpression of nuclear factor erythroid 2-related factor 2 (Nrf2) reversed the effects of metformin in the cisplatin sensitivity of gastric cancer by inhibiting cell viability and metabolic reprogramming and promoting cell apoptosis and oxidative stress. Metformin activated p53 and AMPK pathways in cisplatin-induced NCI-N87 cells, which were reversed by upregulating Nrf2. BAY-3827 (AMPK inhibitor) or p-nitro-Pifithrin-α (p53 inhibitor) treatments also reversed the effects of metformin increased the cisplatin sensitivity of gastric cancer by inhibiting cell viability and metabolic reprogramming and promoting cell apoptosis and oxidative stress. These results suggest that metformin significantly increases chemosensitivity of gastric cancer to cisplatin by inhibiting Nrf2 expression and metabolic reprogramming and activating oxidative stress and the pathway of p53 and AMPK.