The growing demand for sustainable materials has spurred significant advancements in nanotechnology, particularly in developing green nanocomposites. These green nanocomposites are used to address environmental issues that are currently in urgent need, offering solutions that combine ecological responsibility with high performance. This chapter provides a comprehensive overview of sustainable nanomaterials, focusing on the synthesis and application of nanocomposites. The fundamental principles of sustainable nanomaterial design, including life cycle assessment and green chemistry approaches, are discussed in this chapter. A diverse array of bio-based and renewable resources is used as matrix materials in these nanocomposites, such as chitosan, cellulose, and alginate, highlighting their characteristics and benefits over conventional petroleum-based materials. Various environmentally friendly synthesis techniques, such as microbial-assisted processes that use the natural capability of microorganisms, plant-mediated ones that utilize plant extract as capping and reducing agents, and enzyme-catalyzed reactions that offer high specificity and mild reaction conditions, are discussed in detail. The use of ionic liquids as green solvents is also discussed, showcasing the potential to replace volatile organic compounds in nanocomposite synthesis. The characterisation methods used to assess the thermal, mechanical, and structural properties of green nanocomposites, as well as techniques for evaluating their biodegradability and environmental impact, are also discussed. This study analyses green nanocomposites that provide essential insights for material scientists, researchers, and industry professionals, emphasising sustainable nanomaterials’ important role in advancing environmental stewardship and technological progress.