Selenium nanoparticles (SeNPs) are considered a promising option for increasing plant resilience to both abiotic and biotic stresses. This chapter provides a detailed discussion of the properties, synthesis methods, and mechanisms by which SeNPs protect plants. The unique dual role of SeNPs as a nutrient delivery system and stress reducer is highlighted, especially their ability to lessen the effects of abiotic stresses such as drought, salinity, and temperature extremes, as well as biotic stresses like pathogen and pest attacks. SeNPs enhance plant metabolism by promoting photosynthesis, activating antioxidant enzymes, and reducing oxidative stress, thereby supporting overall plant health. Their role in improving stress tolerance is also reflected in their influence on osmotic regulation, ion balance, and immune system activation. Additionally, SeNPs have shown synergistic effects when combined with plant growth regulators (PGRs) and other nanoparticles, offering a comprehensive approach to crop improvement. The chapter also explores environmentally friendly green production methods for SeNPs, which serve as sustainable alternatives to traditional techniques, along with regulatory and safety considerations for their agricultural use. In conclusion, SeNPs hold great potential for advancing sustainable agriculture by boosting crop resistance and yields against environmental and biological challenges.