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Telmisartan, an angiotensin II type 1 receptor blocker with established anti-inflammatory and antihypertensive properties, has been reported to inhibit tumor cell proliferation, yet its impact on the tumor immune microenvironment remains poorly understood. In this study, we evaluated the immunomodulatory effects of telmisartan using a syngeneic MC38 colorectal cancer model in C57BL/6 mice. Daily intragastric administration of telmisartan significantly suppressed tumor growth and reduced endpoint tumor weight compared with controls. To elucidate the underlying mechanisms, we performed single-cell RNA sequencing on tumor-infiltrating CD45⁺ immune cells and revealed a macrophage-dominated immune landscape comprising multiple transcriptionally distinct subclusters. Telmisartan broadly downregulated pro-tumoral and M2-associated macrophage programs, including decreased expression of genes such as Mrc1 and Spp1, while also suppressing cell proliferation-related pathways. In contrast to its overall suppressive impact on macrophages, telmisartan increased the proportion of cytotoxic CD8⁺ T cells, reduced regulatory T cell counts, and enhanced major histocompatibility complex class I antigen presentation, consistent with an immune-activating effect. These results indicate that telmisartan reshapes the colorectal tumor immune microenvironment by simultaneously attenuating tumor-promoting macrophage activity and augmenting cytotoxic T cell responses. Overall, this study provides a single-cell framework to understand how angiotensin receptor blockade reshapes tumor-infiltrating immune programs, highlighting the translational potential of repurposing telmisartan for novel cancer immunotherapy strategies.