T cell-based immunotherapies, such as CAR-T and TCR-T cell therapies, have shown remarkable success owing to their high target specificity and durable efficacy. However, their clinical efficacy is known to be limited by the immunosuppressive tumor microenvironment (TME). In this study, we focused on platelets as one of the immunosuppressive components in the TME. Platelets can extravasate from leaky tumor vessels and infiltrate into tumor tissues, where they directly interact with various types of cells in the tumor tissue. In addition to direct cell-cell interactions, platelets can influence tumor progression and modulate the function of immune cells through the released factors such as EGF, PDGF, and TGF-beta upon platelet activation. In this study, we sought to assess the effect of platelets to the cytotoxic T lymphocytes (CTL). To assess this effect, we performed co-culture experiments of CTLs and tumor cells with human platelets. For the CTLs, we used cancer antigen WT1-specific T cells (CTL3-3) re-differentiated from iPS cells. Cytotoxicity of CTL3-3 was evaluated by co-incubate with HLA matched cancer cells expressing luciferase. After incubation of CTL3-3 with cancer cells with or without WT1 peptide, antigen-specific cytotoxicity was evaluated with luciferase assay. When platelets derived from healthy donors were added to this co-culture system, antigen-specific cytotoxicity was significantly suppressed with IFN-gamma release suppression. This result indicated that platelets inhibited CTL3-3 activity. Interestingly, supernatant from activated platelets by collagen I treatment, partially suppressed the CTL3-3 cytotoxicity, suggesting that intact platelets interaction contribute significantly to the inhibitory effect. We then evaluated the direct interaction of platelets with cancer cells, and CTL3-3. Flow cytometry analysis revealed that platelets, which express the activation marker CD62p, preferentially adhered to CTL3-3. Immunofluorescence and electron microscopy analysis further confirmed this adhesion. Both flow cytometry and quantification using fluorescence images demonstrated that neutralizing antibodies against CD62p or its known binding partner, PSGL-1 significantly reduced platelets-CTL3-3 adhesion, suggesting that activated platelets adhere to CTL3-3 through CD62p and PSGL-1. In conclusion, our findings revealed a mechanism by which platelets inhibit antigen-specific CTL activity through direct adhesion, in addition to the soluble factor-mediated killing activity suppression of CTLs. These results suggest that targeting platelet-CTL interaction might enhance the efficacy of T cell-based immunotherapies and provide a potential strategy to overcome immunosuppressive barriers within the TME.