Blockade of the protein–protein interaction between the transmembrane protein programmed cell death protein 1 (PD‐1) and its ligand PD‐L1 has emerged as a promising immunotherapy for treating cancers. Using the technology of mirror‐image phage display, we developed the first hydrolysis‐resistant D‐peptide antagonists to target the PD‐1/PD‐L1 pathway. The optimized compound DPPA‐1 could bind PD‐L1 at an affinity of 0.51 μM in vitro. A blockade assay at the cellular level and tumor‐bearing mice experiments indicated that DPPA‐1 could also effectively disrupt the PD‐1/PD‐L1 interaction in vivo. Thus D‐peptide antagonists may provide novel low‐molecular‐weight drug candidates for cancer immunotherapy. Protein chemical synthesis and mirror‐image phage display were combined to develop a proteolysis‐resistant D‐peptide antagonist (DPPA‐1) which targets the immune checkpoint protein PD‐L1 (the ligand for PD‐1, the programmed cell death protein 1). DPPA‐1 was found to inhibit the PD‐1/PD‐L1 protein–protein interaction at the cellular level. IgV=immunoglobulin‐like variable.