<p>Selenium (Se), a trace element with dual roles as a micronutrient and toxicant, occupies a unique niche in plant physiology and agricultural science. While not essential for plant growth, selenium’s chemical resemblance to sulfur (S) enables its integration into metabolic pathways. This integration influences redox homeostasis, stress resilience, and the nutritional quality of crops. Recent advancements in nanotechnology have unlocked selenium’s latent potential through the use of engineered selenium nanoparticles (SeNPs), primarily zero-valent elemental (Se⁰) SeNPs. These nanoparticles mitigate the toxicity risks associated with conventional selenite (Se^IVO₃²⁻) or selenate (Se^VIO₄²⁻) formulations. SeNPs combine enhanced bioavailability with distinctive physicochemical properties. This combination enables enhanced plant growth, stress tolerance, nutrient uptake, and seed quality. Green synthesis methods using plant extracts offer eco-friendly routes to produce stable, biocompatible SeNPs with controlled size and morphology. In crops, SeNPs modulate metabolic pathways and improve photosynthetic pigments. They also synergize with S compounds to boost yield and nutritional content. Additionally, SeNPs contribute to sustainable agriculture by mitigating biotic and abiotic stresses. They enhance food safety through antimicrobial properties in packaging and animal nutrition. Despite their promise, careful dose management is critical due to the narrow therapeutic window of Se and potential toxicity at higher concentrations. This review explores the latest SeNPs synthesis, selenium’s paradoxical role in plant systems, and its agrobiological impacts. Beyond applications, the review identifies critical methodological gaps in existing research, proposes a characterization set for SeNPs studies, and introduces a comprehensive framework for future research priorities.</p> Graphical abstract <p></p>