Reactive oxygen species (ROS) are toxic by-products of aerobic cellular metabolism. However, ROS conduct multiple functions, and specific ROS sources can have beneficial or detrimental effects on plant health. This review explores the complex dynamics of ROS in plant defense mechanisms, focusing on their involvement in basal resistance, hypersensitive response (HR), and systemic acquired resistance (SAR). ROS, including superoxide anion (O²⁻), singlet oxygen (¹O₂), hydroxyl radicals (OH), and hydrogen peroxide (H₂O₂), are generated through various enzymatic pathways. They may serve to inhibit pathogen growth while also activating defense-related gene expression as signaling molecules. Oxidative damage in cells is mainly attributed to excess ROS production. ROS produce metabolic intermediates that are involved in various signaling pathways. The oxidative burst triggered by pathogen recognition initiates hyper-resistance (HR), a localized programmed cell death restricting pathogen spread. Additionally, ROS facilitate the establishment of SAR by inducing systemic signaling networks that enhance resistance across the plant. The interplay between ROS and phytohormones such as jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) further complicates this regulatory framework, underscoring the importance of ROS in orchestrating both local and systemic defense responses. Grasping these mechanisms is essential for creating strategies that enhance plant resilience to biotic stresses.