Exercise improves immune checkpoint inhibitor (ICI) efficacy in cancers such as melanoma; however, the mechanisms through which exercise mediates this antitumor effect remain obscure. Here, we identify that the gut microbiota plays a critical role in how exercise improves ICI efficacy in preclinical melanoma. Our study demonstrates that exercise stimulates microbial one-carbon metabolism, increasing levels of the metabolite formate, which subsequently enhances cytotoxic CD8 T cell (Tc1)-mediated ICI efficacy. We further establish that microbiota-derived formate is both sufficient and required to enhance Tc1 cell fate in vitro and promote tumor antigen-specific Tc1 immunity in vivo. Mechanistically, we identify the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) as a crucial mediator of formate-driven Tc1 function enhancement in vitro and a key player in the exercise-mediated antitumor effect in vivo. Finally, we uncover human microbiota-derived formate as a potential biomarker of enhanced Tc1-mediated antitumor immunity, supporting its functional role in melanoma suppression.