PD-L1 molecules on a tumor have attracted attention because PD-L1 on a tumor plays an important role in escape for host immune responses. It has been shown that genistein attenuates immune checkpoint therapy for B16F1 melanoma in mice. We examined the effect of genistein on expression of PD-L1 in B16F1 melanoma cells and found that genistein increases the expression of IFN-γ-induced PD-L1 molecules at both protein and mRNA levels. Genistein increased the mRNA expression of STAT1 and STAT3 in IFN-γ-treated B16F1 cells. We compared the effects of ten types of flavonoids on PD-L1 expression and found that genistein is a strong inducer of PD-L1 expression among the flavonoids.