Protective immunity against pathogens or cancer is mediated by the activation and clonal expansion of antigen-specific naive T cells into effector T cells. To sustain their rapid proliferation and effector functions, naive T cells switch their quiescent metabolism to an anabolic metabolism through increased levels of aerobic glycolysis, but also through mitochondrial metabolism and oxidative phosphorylation, generating energy and signalling molecules^1-3. However, how that metabolic rewiring drives and defines the differentiation of T cells remains unclear. Here we show that proliferating effector CD8⁺ T cells reductively carboxylate glutamine through the mitochondrial enzyme isocitrate dehydrogenase 2 (IDH2). Notably, deletion of the gene encoding IDH2 does not impair the proliferation of T cells nor their effector function, but promotes the differentiation of memory CD8⁺ T cells. Accordingly, inhibiting IDH2 during ex vivo manufacturing of chimeric antigen receptor (CAR) T cells induces features of memory T cells and enhances antitumour activity in melanoma, leukaemia and multiple myeloma. Mechanistically, inhibition of IDH2 activates compensating metabolic pathways that cause a disequilibrium in metabolites regulating histone-modifying enzymes, and this maintains chromatin accessibility at genes that are required for the differentiation of memory T cells. These findings show that reductive carboxylation in CD8⁺ T cells is dispensable for their effector response and proliferation, but that it mainly produces a pattern of metabolites that epigenetically locks CD8⁺ T cells into a terminal effector differentiation program. Blocking this metabolic route allows the increased formation of memory T cells, which could be exploited to optimize the therapeutic efficacy of CAR T cells.