Reinvigoration of antitumor immunity remains an unmet challenge. Our retrospective analyses revealed that cancer patients who took antihistamines during immunotherapy treatment had significantly improved survival. We uncovered that histamine and histamine receptor H1 (HRH1) are frequently increased in the tumor microenvironment and induce T cell dysfunction. Mechanistically, HRH1-activated macrophages polarize toward an M2-like immunosuppressive phenotype with increased expression of the immune checkpoint VISTA, rendering T cells dysfunctional. HRH1 knockout or antihistamine treatment reverted macrophage immunosuppression, revitalized T cell cytotoxic function, and restored immunotherapy response. Allergy, via the histamine-HRH1 axis, facilitated tumor growth and induced immunotherapy resistance in mice and humans. Importantly, cancer patients with low plasma histamine levels had a more than tripled objective response rate to anti-PD-1 treatment compared with patients with high plasma histamine. Altogether, pre-existing allergy or high histamine levels in cancer patients can dampen immunotherapy responses and warrant prospectively exploring antihistamines as adjuvant agents for combinatorial immunotherapy. Display omitted •Histamine binding of HRH1 on macrophages induces an immunosuppressive phenotype•H1-antihistamine treatment enhances immunotherapy response•Allergic reaction promotes immune evasion and resistance to immunotherapy•High histamine and HRH1 levels correlate with poor immunotherapy response in patients Li et al. investigate how cancer cells evade immune attack and resist immunotherapies. Cancer cell-derived or allergy-released histamine binds to HRH1 on tumor-associated macrophages that suppress CD8+ T cell function, accelerate tumor growth, and confer immunotherapy resistance. H1-antihistamines counteract histamine-mediated immunosuppression, reinforce antitumor immunity, and significantly enhance immunotherapy response.