UNLABELLED: Selenium nanoparticles (SeNPs) captivate researchers due to their unique properties and promise in biomedicine. This review explores SeNP synthesis methods, emphasizing how they influence functionality in diverse applications. Chemical, physical, and biological approaches tailor SeNP size, shape, and surface chemistry, impacting their biocompatibility and potential for drug delivery, imaging, and therapy. SeNPs' remarkable electrochemical and optical properties position them for advancements in biosensing and diagnostics. However, challenges in consistent production, large-scale synthesis, and potential toxicity demand attention. We provide a concise analysis of current SeNP research, encompassing synthesis strategies, characterization techniques, and a broad spectrum of biomedical applications, while also addressing ongoing challenges and future directions in this rapidly evolving field. GRAPHICAL ABSTRACT: The multifaceted biological roles of Se NPs encompass orchestrating cellular processes, targeted drug delivery, enhancing chemotherapy efficacy, and providing protective effects against treatment-related toxicity.