Heavy metal soil contamination represents a serious risk to crop productivity, food safety, and ecosystem health. Nano-selenium (Nano-Se), as an emerging nanomaterial, has been widely studied owing to its unique redox properties, biocompatibility, and multiple functionalities in reducing heavy metal toxicity and promoting plant growth. Nano-Se functions as a dual-action biomolecule that reduces the bioavailability of toxic metals, including cadmium (Cd), lead (Pb), and arsenic (As), through adsorption, chelation, and redox transformations. Moreover, it stimulates plant defence responses while increasing antioxidant enzyme activity and nutrient uptake. This chapter summarises the pathways of Nano-Se that have been experienced in detoxifying heavy metals and promoting the growth of plants under stress conditions. It discusses the recent developments in synthesising, characterising, and using Nano-Se in contaminated agroecosystems. Additionally, the chapter explores new avenues of research towards the recommended optimum use of Nano-Se for sustainable agricultural practices in contaminated settings, including potential safety concerns and environmental sustainability aspects.