This review provides an essential insight into the principles of nanotechnology for nano-selenium based materials, focusing on selenium nanoparticles (SeNPs) and selenium nanocomposites (SeNCs), including their synthesis and biomedical applications. The study explores various synthesis methods for nano-selenium, highlighting their potential as pharmacological agents with antimicrobial, anticancer, anti-diabetic, and antioxidant properties. It also discusses methods for assessing toxicity under in vitro and in vivo conditions using simple, cost-effective, and green technologies. In vitro toxicity methods involve selected cell lines to determine cytotoxic effects, while phytotoxicity screening is a simple and reliable method. Animal models such as rats, mice, and zebrafish are widely used to study the toxic effects of nano-selenium. The review emphasizes the importance of understanding the nanotechnology principles of selenium nanomaterials for their diverse applications with high biocompatibility and environmental friendliness. Selenium is a crucial trace element in the human body, essential for selenoproteins, and has various applications in photovoltaic cells, medical devices, and nanotechnology. Selenium nanoparticles have lower toxicity compared to selenite and selenate, and they exhibit improved biocompatibility and degradability, reducing the risk of oncogenesis and aiding in the prevention of tumors and cardiovascular diseases. The FDA recommends daily selenium intake of 53–60 µg for adults. Selenium nanoparticles can enhance the proliferation of healthy human cells and can be synthesized through physical, chemical, and biological methods. The review highlights the potential of nano-selenium in biomedical applications due to their unique properties and low toxicity.This review provides an essential insight into the principles of nanotechnology for nano-selenium based materials, focusing on selenium nanoparticles (SeNPs) and selenium nanocomposites (SeNCs), including their synthesis and biomedical applications. The study explores various synthesis methods for nano-selenium, highlighting their potential as pharmacological agents with antimicrobial, anticancer, anti-diabetic, and antioxidant properties. It also discusses methods for assessing toxicity under in vitro and in vivo conditions using simple, cost-effective, and green technologies. In vitro toxicity methods involve selected cell lines to determine cytotoxic effects, while phytotoxicity screening is a simple and reliable method. Animal models such as rats, mice, and zebrafish are widely used to study the toxic effects of nano-selenium. The review emphasizes the importance of understanding the nanotechnology principles of selenium nanomaterials for their diverse applications with high biocompatibility and environmental friendliness. Selenium is a crucial trace element in the human body, essential for selenoproteins, and has various applications in photovoltaic cells, medical devices, and nanotechnology. Selenium nanoparticles have lower toxicity compared to selenite and selenate, and they exhibit improved biocompatibility and degradability, reducing the risk of oncogenesis and aiding in the prevention of tumors and cardiovascular diseases. The FDA recommends daily selenium intake of 53–60 µg for adults. Selenium nanoparticles can enhance the proliferation of healthy human cells and can be synthesized through physical, chemical, and biological methods. The review highlights the potential of nano-selenium in biomedical applications due to their unique properties and low toxicity.