MAPK inhibitor (MAPKi) therapy in melanoma leads to accumulation of tumor-surface PD-L1/2, which may evade antitumor immunity and accelerate acquired resistance. Here, we discover that the E3 ligase ITCH binds, ubiquitinates, and down-regulates tumor-surface PD-L1/L2 in MAPKi-treated human melanoma cells, thereby promoting T-cell activation. During MAPKi therapy in vivo, melanoma cell-intrinsic ITCH knockdown induced tumor-surface PD-L1, reduced intratumoral cytolytic CD8+ T cells, and accelerated acquired resistance only in immune-competent mice. Conversely, tumor cell-intrinsic ITCH over-expression reduced MAPKi-elicited PD-L1 accumulation, augmented intratumoral cytolytic CD8+ T cells, and suppressed acquired resistance in BrafV600MUT, NrasMUT, or Nf1MUT melanoma and KrasMUT-driven cancers. CD8+ T-cell depletion and tumor cell-intrinsic PD-L1 over-expression nullified the phenotype of ITCH over-expression, thereby supporting an in vivo ITCH-PD-L1-T-cell regulatory axis. Moreover, we identify a small-molecular ITCH activator which suppresses acquired MAPKi-resistance in vivo. Thus, MAPKi-induced PD-L1 accelerates resistance, and a PD-L1-degrading ITCH activator prolongs antitumor response.