Mitochondrial dynamics regulate T cell fate through metabolic programming. Activated effector T (T_E) cells use aerobic glycolysis, while memory T (T_M) cells rely on fatty acid oxidation (FAO). T_M cells have fused mitochondrial networks, whereas T_E cells have fragmented mitochondria. The fusion protein Opa1 is essential for T_M cell development but not T_E cells. Enforcing mitochondrial fusion in T_E cells converts them into T_M-like cells with enhanced antitumor function. Fusion in T_M cells promotes oxidative phosphorylation (OXPHOS) and FAO, while fission in T_E cells reduces ETC efficiency and promotes aerobic glycolysis. Mitochondrial remodeling acts as a signaling mechanism that dictates T cell metabolic programming. Opa1 is necessary for T_M cell formation, but not for FAO. T_M cells have tightly packed cristae with closely associated ETC complexes, supporting efficient OXPHOS and redox balance. T_E cells have looser cristae, leading to reduced ETC efficiency and increased aerobic glycolysis. Mitochondrial fission and fusion regulate cristae remodeling, which influences metabolic adaptations in T cells. T_M cells maintain tight cristae, enabling efficient OXPHOS and long-term survival, while T_E cells have looser cristae, promoting aerobic glycolysis and shorter lifespan. Mitochondrial dynamics control the balance between metabolic pathways and T cell fate. This study highlights the role of mitochondrial dynamics in shaping T cell metabolism and function.Mitochondrial dynamics regulate T cell fate through metabolic programming. Activated effector T (T_E) cells use aerobic glycolysis, while memory T (T_M) cells rely on fatty acid oxidation (FAO). T_M cells have fused mitochondrial networks, whereas T_E cells have fragmented mitochondria. The fusion protein Opa1 is essential for T_M cell development but not T_E cells. Enforcing mitochondrial fusion in T_E cells converts them into T_M-like cells with enhanced antitumor function. Fusion in T_M cells promotes oxidative phosphorylation (OXPHOS) and FAO, while fission in T_E cells reduces ETC efficiency and promotes aerobic glycolysis. Mitochondrial remodeling acts as a signaling mechanism that dictates T cell metabolic programming. Opa1 is necessary for T_M cell formation, but not for FAO. T_M cells have tightly packed cristae with closely associated ETC complexes, supporting efficient OXPHOS and redox balance. T_E cells have looser cristae, leading to reduced ETC efficiency and increased aerobic glycolysis. Mitochondrial fission and fusion regulate cristae remodeling, which influences metabolic adaptations in T cells. T_M cells maintain tight cristae, enabling efficient OXPHOS and long-term survival, while T_E cells have looser cristae, promoting aerobic glycolysis and shorter lifespan. Mitochondrial dynamics control the balance between metabolic pathways and T cell fate. This study highlights the role of mitochondrial dynamics in shaping T cell metabolism and function.