ABSTRACT: Cancers only develop if they escape immunosurveillance, and the success of cancer immunotherapies relies in most cases on their ability to restore effector T-cell functions, particularly IFN-γ production. Revolutionizing the treatment of many cancers, immunotherapies targeting immune checkpoints such as PD1 can increase survival and cure patients. Unfortunately, although immunotherapy has greatly improved the prognosis of patients, not all respond to anti-PD1 immunotherapy, making it crucial to identify alternative treatments that could be combined with current immunotherapies to improve their effectiveness. Here, we show that iron supplementation significantly boosts T-cell responses in vivo and in vitro. This boost is associated with a metabolic reprogramming of T cells in favor of lipid oxidation. We also found that the "adjuvant" effect of iron led to a marked slowdown of tumor-cell growth after tumor-cell line transplantation in mice. Specifically, our results suggest that iron supplementation promotes anti-tumor responses by increasing IFN-γ production by T cells. In addition, iron supplementation considerably improves the efficacy of anti-PD1 cancer immunotherapy in mice. Finally, our study suggests that, in cancer patients, the quality and efficacy of the anti-tumor response following anti-PD1 immunotherapy may be modulated by plasma ferritin levels. In summary, our results suggest the benefits of iron supplementation on the reactivation of anti-tumor responses and support the relevance of a fruitful association between immunotherapy and iron supplementation.