Despite the clinical success of immune checkpoint blockade (ICB), in certain cancer types, most patients with cancer do not respond well. Furthermore, in patients for whom ICB is initially successful, this is often short-lived because of the development of resistance to ICB. The mechanisms underlying primary or secondary ICB resistance are incompletely understood. Here, we identified preferential activation and enhanced suppressive capacity of regulatory T cells (T _reg cells) in αPD-L1 therapy–resistant solid tumor–bearing mice. T _reg cell depletion reversed resistance to αPD-L1 with concomitant expansion of effector T cells. Moreover, we found that tumor-infiltrating T _reg cells in human patients with skin cancer, and in patients with non–small cell lung cancer, up-regulated a suppressive transcriptional gene program after ICB treatment, which correlated with lack of treatment response. αPD-1/PD-L1–induced PD-1 ⁺ T _reg cell activation was also seen in peripheral blood of patients with lung cancer and mesothelioma, especially in nonresponders. Together, these data reveal that treatment with αPD-1 and αPD-L1 unleashes the immunosuppressive role of T _reg cells, resulting in therapy resistance, suggesting that T _reg cell targeting is an important adjunct strategy to enhance therapeutic efficacy.