Melatonin (N-acetyl-5-methoxytryptamine) is a highly conserved molecule that is not exclusive to the animal kingdom and its presence in plants has been confirmed by numerous studies. Melatonin has delayed flowering in some day-sensitive plants. When spinach enters the reproduction phase, its nutritional value is markedly reduced. This experiment was conducted to evaluate the effects of melatonin on some growth and flowering traits of spinach based on a completely randomized design with 8 treatments and 12 replications. Treatments included seed priming and foliar application of melatonin at concentrations of 0, 25, 50, and 100 mg L^-1. The results showed that leaf area, fresh and dry weight of shoots, the number of lateral roots, chlorophyll content, and the amount of endogenous melatonin in seed-primed plants were increased by 14.8, 21.8, 22, 29.86, 64.78, and 97%, respectively. Flowering time of plants was delayed by exogenous application of melatonin. Plants derived from seeds primed with 100 mg L^-1 of melatonin showed greater days to flowering with higher number of leaves at this stage. This delay in flowering was confirmed at the microscopic level by examining the meristems at four-leaf stage. Based on the results of this study, it can be concluded that melatonin delayed the transition to the reproductive phase and left the plant in the vegetative phase by enhancing vegetative growth of the spinach plant.