<p>Inhibitors that target the programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) immune checkpoint pathway have revolutionized cancer immunotherapy; however, many patients exhibit a limited response. In this study, we examined the potential of teriflunomide (TER), an FDA-approved drug for multiple sclerosis, as a novel immune checkpoint modulator for treating colorectal cancer (CRC). We determined the effect of TER on PD-L1 expression in human CRC cell lines, its direct binding to PD-1, and its impact on CD8⁺ T-cell function. Antitumor activity was determined in vivo using a humanized mouse model of hPD-1 knock-in mice implanted with hPD-L1 expressing MC38 tumor cells. TER treatment reduced PD-L1 expression in CRC cells and disrupted the PD-1/PD-L1 interaction directly. In vivo, TER significantly suppressed tumor growth without systemic toxicity, and enhanced the infiltration and activation of CD8⁺ T cells within tumors, as evidenced by increased granzyme B expression. Moreover, the antitumor efficacy of TER was abolished by the depletion of CD8⁺ T cells, which indicated its dependency on this cell population. These findings highlight TER as a promising immune checkpoint modulator that targets the PD-1/PD-L1 axis to promote CD8⁺ T-cell-mediated antitumor immunity. Because of its established safety profile, TER is a readily translatable therapeutic for enhancing cancer immunotherapy in CRC.</p>