Plants frequently encounter biotic stresses due to pathogens (including viruses, fungi, oomycetes, bacteria, and insects), whereas the interaction between plants and pathogens represents an ongoing evolutionary battle, as both strive to outcompete each other in natural ecosystems. To counteract these threats, to minimize pest and pathogen attacks, plants have developed intricate resistance mechanisms. Plant hormones such as melatonin, salicylic acid, jasmonates, abscisic acid, gibberellins, auxins, brassinosteroids, ethylene, cytokinins, and melatonin operate within a well-coordinated network that underpins plant resistance to pests and diseases shows its role as the major driving factor of numerous plant physiological processes. Melatonin (N-acetyl-5-methoxytryptamine; derived from tryptophan) is synthesized by various organisms, including bacteria, plants, animals, and fungi. Since its identification in plants, significant progress has been made in understanding phytomelatonin functions. This multifunctional molecule, often referred to as a master regulator, influences certain mechanisms of plants (such as seeds and fruits ripening, plant growth, parthenocarpy, rooting, flowering, leaf senescence, biomass production, and photosynthetic efficiency). One of its most important roles is to protect against biotic stress. This chapter reviews the metabolism and biosynthesis of melatonin in plants, the defensive mechanism at the cellular level against diseases, and biotic stresses. Moreover, the interaction of melatonin with other plant substances integrated the melatonin-plant biocontrol model against various plant defense processes. In the end, knowledge regarding melatonin as an antiviral, antibacterial, antifungal, and pesticide agent against various plant pathogens.