<i>In vivo</i>, melatonin treatment suppressed tumor growth, increased CD8+ T-cell infiltration, and decreased PD-L1 expression and the number of FOXP3+ regulatory T cells in the tumor microenvironment.<h4>Conclusions</h4>Melatonin suppresses TNBC progression by inhibiting proliferation and migration and by modulating the immune microenvironment through the FAK-PD-L1 axis. The gene discussed is FOXP3; the disease is neoplasm.