Earlier melatonin was considered a multifunctional molecule in animal physiology but has lately been recognized as playing an important role in enhancing plant immunity against fungal pathogens. This chapter now deals with the involvement of melatonin in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI), two major mechanisms that protect plants against fungal diseases. PTI is a non-specific defense that depends upon the recognition of PAMPs via PRRs, whereas ETI is a specific resistance by the plant to the specific pathogen mediated by an R gene-dependent recognition mechanism of the effectors. It plays a key role in both immunity layers through modulating signaling pathways, activation of defense-related genes, and balancing the production of reactive oxygen species (ROS). Melatonin enhances PTI by influencing PAMP perception and subsequent pattern recognition receptor (PRR) activation, whereas in ETI, it is contributed by amplifying the expression of R genes and promoting systemic acquired resistance (SAR). Melatonin plays a crucial role in strengthening plant defenses by working in synergy with both PTI and ETI, making it a key player in integrating these immune responses for a more robust defense system. In agriculture, melatonin holds great potential—not only as a natural, phytohormone-like modulator that supports sustainable crop treatments but also as a powerful tool for boosting plant immunity and minimizing damage caused by fungal diseases.