Melatonin (MT) functions in removing reactive oxygen species (ROS) and delaying plant senescence, thereby acting as an antioxidant; however, the molecular mechanism underlying the specific action of MT is unclear. Herein, we used the mutant plants carrying the MT decomposition gene melatonin 3‐hydroxylase (M3H) in tomato to elucidate the specific mechanism of action of MT. SlM3H‐OE accelerated senescence by decreasing the content of endogenous MT in plants. SlM3H is a senescence‐related gene that positively regulates aging. MT inhibited the expression of the senescence‐related gene SlCV to scavenge ROS, induced stable chloroplast structure, and delayed leaf senescence. Simultaneously, MT weakened the interaction between SlCV and SlPsbO/SlCAT3, reduced ROS production in photosystem II, and promoted ROS elimination. In conclusion, MT regulates ROS homeostasis and delays leaf aging in tomato plants through SlCV expression modulation.