A series of novel compounds bearing a cyclopropyl linkage were designed, synthesized, and evaluated as programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) inhibitors. An optimized compound (1S,2S)-A25 showed potent inhibitory activity against the PD-1/PD-L1 interaction (IC50 = 0.029 μM) with a selected binding affinity with PD-L1 (KD = 1.554 × 10-1 μM). Additionally, under the co-culture with H460/Jurkat cells, (1S,2S)-A25 can reduce the survival of H460 cells in a concentration-dependent way. A liver microsomal assay revealed that (1S,2S)-A25 had favorable metabolic stability. Furthermore, (1S,2S)-A25 displayed favorable pharmacokinetic properties (oral bioavailability of 21.58%) and potent antitumor potency in a LLC1 lung carcinoma model without observable side effects. Data from the flow cytometry and enzyme-linked immunosorbent assays demonstrated that (1S,2S)-A25 suppressed the tumor growth by activating the immune microenvironment. Our study suggests that (1S,2S)-A25 is a promising lead compound for the further development of PD-1/PD-L1 inhibitors.