<p>The programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) pathway, a pivotal immune checkpoint, enables tumor immune evasion, and its blockade is fundamental to cancer immunotherapy. The development of small-molecule agents targeting the PD-1/PD-L1 pathway offers a promising strategy for enhancing antitumor immunity. In this study, we screened an in-house compound library using RKO cells to discover novel PD-L1 downregulators. MS1-96 was identified as a potent PD-L1 degrader that promotes lysosome-dependent PD-L1 degradation. Furthermore, MS1-96 effectively reduced PD-L1 protein levels across multiple colorectal cancer (CRC) cell lines. By disrupting the PD-1/PD-L1 pathway, MS1-96 enhances CD8⁺ T cell-mediated killing of carcinoma cells and exerts dose-dependent antitumor effects in C57BL/6 mice bearing MC38 CRC xenografts, resulting in significant tumor growth inhibition after oral administration for 10 d (100, 200, or 400 mg·kg⁻¹·d⁻¹). Mechanistic studies revealed that Huntingtin interacting protein 1-related (HIP1R) plays an indispensable role in MS1-96-driven PD-L1 degradation, and HIP1R knockdown abolishes MS1-96’s ability to degrade PD-L1. MS1-96 directly binds to PD-L1 with a <i>K</i>_D of 2.58 μM and enhances the interaction between HIP1R and PD-L1, thereby altering the intracellular trafficking of PD-L1 within clathrin-coated vesicles. This leads to reduced transport of PD-L1 to recycling endosomes and increased delivery to late endosomes and lysosomes for degradation. Furthermore, MS1-96 induces abnormal N-glycosylation of PD-L1, destabilizing the protein and hastening its lysosome-mediated degradation. Moreover, MS1-96 effectively enhances the antitumor efficacy of PD-1 antibodies in MC38 CRC models. These findings indicate that MS1-96 offers a potential strategy for advancing tumor immunotherapy.</p>