Inhibition of the programmed cell death-1 (PD-1)/programmed cell death-ligand 1 (PD-L1) interaction using small-molecule inhibitors is an emerging immunotherapeutic approach. A novel series of [1,2,4]triazolo[4,3- a]pyridines were designed and found to be potent inhibitors of the PD-1/PD-L1 interaction. Among them, compound A22 exhibited the most potent activity, as assessed by homogenous time-resolved fluorescence assay, with an IC50 of 92.3 nM. Furthermore, A22 dose-dependent elevated interferon-γ production in a co-culture model of Hep3B/OS-8/hPD-L1 and CD3 T cells. We concluded that A22 is a promising lead compound for the development of inhibitors of the PD-1/PD-L1 interaction. In addition, we explored the structure-activity relationships of the newly synthesized [1,2,4]triazolo[4,3- a]pyridines and demonstrated that a ring fusion strategy can be employed for designing analogues of the Bristol-Myers Squibb chemical series. These studies pave the way for future drug design.