Recently, Siglec-15 has been proved as a novel immune suppressor and a potential target for normalization cancer immunotherapy, which is non-redundant to the well-known PD-L1/PD-1 pathway. Herein, anti-Siglec-15 mAb, a monoclonal antibody (mAb) with a high affinity against Siglec-15, was prepared.The engineered CHO-K1 Siglec-15 cell line was constructed to heterologously expressed Siglec-15 for the affinity test with the mAb. Antigens Siglec-15-mIgG and Siglec-15-his were recombinantly expressed by 293F cells and purified by high-performance liquid chromatography (HPLC). Hybridoma cell line against Siglec-15 was prepared and validated by enzyme-linked immunoabsorbant assay (ELISA) and fluorescent-activated cell sorting (FACS). Finally, the anti-Siglec-15 mAb was produced, purified, and confirmed by SDS-PAGE, ELISA, and FACS.The EC50 of the anti-Siglec-15 mAb with Siglec-15 is 76.65 ng/mL, lower than that of the positive control 5G12 (90.7 ng/mL), indicating a high affinity of the anti-Siglec-15 mAb. In vitro and in vivo studies verified that the anti-Siglec-15 mAb blocks the Siglec-15-mediated suppression of T cell and moderately prevents the tumor growth.The anti-Siglec-15 mAb can be considered as an effective immunotherapy for tumor suppression.The anti-Siglec-15 mAb prepared in this study is useful as an immune checkpoint inhibitor against Siglec-15 for normalization cancer immunotherapy. This immunotherapy provides an alternative treatment for cancer patients who are refractory to the well-known PD-L1/PD-1-targeting therapies.Copyright: © Whioce Publishing Pte. Ltd.