Abstract: To investigate preservative effects of exogenous melatonin (MT) treatment on pitaya fruit, different concentrations (0.05, 0.10, and 0.50 mmol/L) of MT were used to treat harvested pitaya fruit, and the storage quality and scavenging capacity of reactive oxygen species (ROS) of MT treatment were measured. The results showed that preservative effects of MT depended on treatment concentration, and 0.10 mmol/L MT treatment significantly enhanced storage quality in relative to 0.05 mmol/L and 0.50 mmol/L concentrations. 0.10 mmol/L MT treatment effectively inhibited scale yellowing, reduced weight loss, delayed the decline in total soluble solids and titratable acid content, as well as minimized decay rate. Furthermore, 0.10 mmol/L MT treatment significantly enhanced the activities of catalase, peroxidase, and ascorbate peroxidase, along with the total antioxidant capacity, thereby enhancing the scavenging ability to DPPH· and superoxide anion radicals. The above results suggested that MT treatment enhanced antioxidant enzyme activity, facilitating the timely removal of ROS generated during fruit senescence and reducing oxidative damages caused by ROS.