Marigold (Calendula officinalis L.) is one of the important, popular and widely used seasonal plant species in Iran. Besides its medicinal and nutritional uses, it plays a prominent role in landscape design and urban green spaces. Due to its antioxidant properties and bioactive compounds, this plant has received increasing attention in various science fields such as horticultural and pharmaceutical sciences. Melatonin, as a plant growth regulator, is known for its multifaceted effects on growth regulation, incrising resistance to stress factors, enhancing photosynthetic capacity, and improving metabolic activities. This substance plays a vital role in regulating physiological and biochemical reactions in plants, which can lead to enhanced quality and efficiency of ornamental products. The present study aimed to investigate the effects of different concentrations of melatonin on the amount and combination of photosynthetic pigments, including chlorophyll a and b, total chlorophyll, and carotenoids, in marigold. The experiment was conducted under a completely controlled greenhouse conditions in autumn 2024. Melatonin treatments included concentrations of 0, 100, 150, and 200 micromolar, which were applied as foliar spray at two different growth stages: one, when plant had 6-8 leaves and the next, two weeks later to investigate its effect on the growth and photosynthetic pigments of marigold. The results of the analyses showed that the 150 micromolar melatonin treatment had the greatest effect and significantly increased the level of plant pigments of chlorophyll a, chlorophyll b and total chlorophyll, and the 100 micromolar melatonin treatment caused the most significant increase in carotenoids pigmet; so that chlorophyll a, chlorophyll b , total chlorophyll and carotenoid increased by 77.34%, 87.41%, 81.72%, and 74.80%, respectively, compared to the control sample. This significant enhancement in photosynthetic pigments indicates improved photosynthetic system performance and enhanced light absorption and energy conversion capacity in the plants. Overall, the result of findings show the importance of melatonin as an effective growth regulator in sustainable agriculture and green space management. Optimal application of this plant growth regulator can increase the productivity and quality of ornamental flowers while also promoting eco-friendly and low-cost solutions to improve plant performance and longevity.