The convergence of nanotechnology with biomedicine and diagnostics heralds a transformative era, particularly with the introduction of nanomaterials. Within this evolving landscape, plant-based nanomaterials have emerged as a significant alternative, aligning with the global imperative for sustainable and biocompatible healthcare technologies. This chapter explores the distinctive properties of plant-based nanomaterials and their growing role in biomedicine and nanodiagnostics. The shift toward these materials addresses critical challenges, including climate change and the need for sustainable practices in medicine. The healthcare industry, traditionally reliant on hazardous, toxic, or difficult-to-dispose-of materials, requires decarbonization. Plant-based nanomaterials, derived from renewable, nontoxic, and biodegradable sources, offer a compelling solution. This chapter centers on five plant-based nanoparticles: nanocellulose, lignin nanoparticles, alginate nanoparticles, starch nanoparticles, and pectin nanoparticles. Selected for their potential and established medical applications, these nanoparticles are inherently functional and serve as scaffolds for further modifications, enhancing their suitability for biomedical applications. Moreover, this chapter delves into hybridization, surface modification, and other modification processes of plant nanomaterials, pivotal for meeting specific biomedical needs such as targeted drug delivery, improved imaging, or enhanced therapeutic efficacy. The discussion extends to the applications of these materials in drug delivery systems, nanodiagnostics, 3D/4D bioprinting, and tissue engineering, underscoring their potential to stand at the forefront of a paradigm shift in healthcare technologies, promising a future where sustainability and medical efficacy converge.