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Metformin, a medication known for its anti-glycemic properties, also demonstrates potent immune system activation. In our study, using a 4T1 breast cancer model in BALB/C WT mice, we examined metformin’s impact on the functional phenotype of multiple immune cells, with a specific emphasis on natural killer T (NKT) cells due to their understudied role in this context. Metformin administration delayed the appearance and growth of carcinoma. Furthermore, metformin increased the percentage of IFN-γ⁺ NKT cells, and enhanced CD107a expression, as measured by MFI, while decreasing PD-1⁺, FoxP3⁺, and IL-10⁺ NKT cells in spleens of metformin-treated mice. In primary tumors, metformin increased the percentage of NKp46⁺ NKT cells and increased FasL expression, while lowering the percentages of FoxP3⁺, PD-1⁺, and IL-10-producing NKT cells and KLRG1 expression. Activation markers increased, and immunosuppressive markers declined in T cells from both the spleen and tumors. Furthermore, metformin decreased IL-10⁺ and FoxP3⁺ Tregs, along with Gr-1⁺ myeloid-derived suppressor cells (MDSCs) in spleens, and in tumor tissue, it decreased IL-10⁺ and FoxP3⁺ Tregs, Gr-1⁺, NF-κB⁺, and iNOS⁺ MDSCs, and iNOS⁺ dendritic cells (DCs), while increasing the DCs quantity. Additionally, increased expression levels of MIP1a, STAT4, and NFAT in splenocytes were found. These comprehensive findings illustrate metformin’s broad immunomodulatory impact across a variety of immune cells, including stimulating NKT cells and T cells, while inhibiting Tregs and MDSCs. This dynamic modulation may potentiate its use in cancer immunotherapy, highlighting its potential to modulate the tumor microenvironment across a spectrum of immune cell types.