Environmental nutrient availability influences T cell metabolism, impacting T cell function and shaping immune outcomes. Here, we identified ketone bodies (KBs)—including β-hydroxybutyrate (βOHB) and acetoacetate (AcAc)—as essential fuels supporting CD8⁺ T cell metabolism and effector function. βOHB directly increased CD8⁺ T effector (Teff) cell cytokine production and cytolytic activity, and KB oxidation (ketolysis) was required for Teff cell responses to bacterial infection and tumor challenge. CD8⁺ Teff cells preferentially used KBs over glucose to fuel the tricarboxylic acid (TCA) cycle in vitro and in vivo. KBs directly boosted the respiratory capacity and TCA cycle-dependent metabolic pathways that fuel CD8⁺ T cell function. Mechanistically, βOHB was a major substrate for acetyl-CoA production in CD8⁺ T cells and regulated effector responses through effects on histone acetylation. Together, our results identify cell-intrinsic ketolysis as a metabolic and epigenetic driver of optimal CD8⁺ T cell effector responses.