Antibody-drug conjugate (ADC) and immune checkpoint blockade (ICB) offer promising approaches for cancer treatment. Here, we describe an ADC constructed by conjugating anti-PD-L1 THIOMAB with a bifunctional immunomodulator D18 via a redox-cleavable linker. The resulting ADC HE-S2 not only triggers a potent antitumor immune response by blocking the PD-1/PD-L1 interaction and activating the Toll-like receptor 7/8 (TLR7/8) signaling pathway but also upregulates its targeted PD-L1 expression via epigenetic regulation and/or IFN-γ induction, thus conferring more sensitivity to the PD-1/PD-L1 blockade. We identify that ADC HE-S2 treatment could lead to more pronounced tumor suppression than the treatment of D18 in combination with the anti-PD-L1 antibody. Accordingly, this study provides a novel ADC strategy to enhance the antitumor immune response to ICB therapy.