PD-L1-IN-1 is a potent PD-L1 inhibitor with an IC50 of 115 nM. PD-L1-IN-1 strongly binds with the PD-L1 protein and challenged it in a co-culture of PD-L1 expressing cancer cells (PC9 and HCC827 cells) and peripheral blood mononuclear cells enhanced antitumor immune activity of the latter. PD-L1-IN-1 significantly increased interferon γ release and apoptotic induction of cancer cells, with low cytotoxicity in healthy cells.