Onion (Allium cepa) is globally recognized not only as a culinary staple but also as a significant source of nutraceuticals and bioactive compounds. While the bulb has been extensively studied for its therapeutic properties, the green leaves remain underexplored despite their rich phytochemical profile. Onion leaves are a valuable reservoir of flavonoids, phenolic acids, saponins, organosulfur compounds, and essential vitamins and minerals, contributing to their remarkable antioxidant, antimicrobial, anti-inflammatory, and antidiabetic properties. This chapter highlights the nutraceutical potential of onion leaves by systematically analyzing their bioactive constituents, modes of action, and associated health benefits. Special emphasis is given to their role in reducing oxidative stress, modulating gut microbiota, regulating blood glucose levels, improving cardiovascular health, and enhancing immunity. Furthermore, the chapter explores their application in functional foods, herbal formulations, and pharmaceutical products, with an outlook on sustainable utilization of agricultural by-products. By integrating scientific evidence and practical insights, this work provides a comprehensive overview of onion leaves as a cost-effective, eco-friendly, and health-promoting resource. It also identifies research gaps and future perspectives to encourage innovative applications of Allium cepa leaves in nutraceutical and health industries.