Gastric cancer is one of the most common malignant tumor types worldwide, with a high morbidity and associated mortality. The interaction between gastric cancer cells and their microenvironment has a significant role in their maintenance and progression. Gastric tumor‑associated fibroblasts (TAFs) are among the major regulators of the gastric cancer microenvironment. Metformin, a classical anti‑diabetic drug, is known to prevent cancer progression. However, the effect of metformin on gastric TAFs and TAF‑associated cancer progression has remained to be elucidated. In the present study, TAFs were isolated from gastric cancer patients, pre‑treated with metformin and then co‑cultured with gastric cancer cell lines. It was demonstrated that pre‑treatment with 200 µM metformin reduced the stimulatory effect of TAFs on the proliferation of gastric cancer cells in co‑culture, suggesting that metformin impairs the tumor‑promoting role of TAFs. Using tandem mass tags‑based quantitative proteomic analysis, it was identified that metformin significantly affected the secretion of 32 proteins (14 upregulated and 18 downregulated) in the culture medium of gastric TAFs. Among these proteins, calmodulin‑like protein 3 (Calml3) was 2.88‑fold upregulated in the culture medium of gastric TAFs after metformin treatment and a further experiment using recombinant Calml3 indicated its suppressive effect on the clonogenicity of gastric cancer cells. It was concluded that metformin suppresses gastric cancer through stimulating Calml3 secretion from TAFs, which represents a novel anticancer mechanism of metformin.