Selenium (Se) is an essential trace element for many organisms, including mammals, bacteria, and certain green algae. It plays a crucial role in human and animal health by activating antioxidant defence systems. In the past decade, the perception of selenium has shifted; it is now recognized as being present in the human body in amounts of 10–15 mg, with an adequate daily intake of about 60 µg. Although selenium is toxic at high concentrations, it significantly contributes to plant growth and resistance to abiotic stresses like salinity, drought, and oxidative stress. Plants primarily absorb selenium as selenate and selenite through root high-affinity membrane transporters, which usually mediate the influx of sulfate and phosphate ions. Selenium is then incorporated into the sulphur assimilation pathway, forming Se-amino acids such as Se-cysteine (SeCys) and Se-methionine (SeMet). The use of herbal products has positive effects on treating diseases caused by free radicals. Medicinal and aromatic plants, rich in antioxidants including selenium, can enhance plant growth and flavour. The elemental composition of plants, along with their secondary metabolites, is vital for their biological activity and human health. Special attention is given to selenium for its powerful antioxidant properties and immune-modulating effects, being a component of antioxidant enzymes. Selenium enhances the morpho-physiological and phytochemical properties of plants by stimulating the production of secondary metabolites and boosting both enzymatic and non-enzymatic antioxidant capacities. Foliar application of selenium is a common technique to increase selenium content in edible parts and has shown positive effects on plant growth characteristics. The beneficial role of selenium in human health and plant physiology underscores its importance and potential applications in the food and health industry.