Abstract Colorectal carcinogenesis coincides with immune cell dysfunction. Metformin has been reported to play a role in stimulating anti-tumor immunity, suggesting it could be used to overcome immunosuppression in colorectal cancer (CRC). Herein, using single-cell RNA sequencing, we showed that metformin remodels the immune landscape of CRC. In particular, metformin treatment expanded the proportion of CD8+ T cells and potentiated their function. Analysis of the metabolic activities of cells in the CRC tumor microenvironment (TME) at a single-cell resolution demonstrated that metformin reprogrammed tryptophan metabolism, which was reduced in CRC cells and increased in CD8+ T cells. Untreated CRC cells outcompeted CD8+ T cells for tryptophan, leading to impaired CD8+ T cell function. Metformin in turn reduced tryptophan uptake by CRC cells, thereby restoring tryptophan availability for CD8+ T cells and increasing their cytotoxicity. Metformin inhibited tryptophan uptake in CRC cells by downregulating MYC, which led to a reduction in the tryptophan transporter SLC7A5. This work highlights metformin as an essential regulator of T-cell antitumor immunity by reprogramming tryptophan metabolism, suggesting it could be a potential immunotherapeutic strategy for treating CRC.