Satureja hortensis L. is a medicinal and aromatic plant widely cultivated in Iran and utilized in the food, cosmetic, and pharmaceutical industries due to its rich content of secondary metabolites. Selenium is an essential trace element for humans, animals, and microorganisms. Considering the importance of medicinal plant cultivation and the role of selenium in human health, the present study was conducted to evaluate the effects of selenium on the growth and biochemical characteristics of summer savory. The experiment was carried out under greenhouse conditions at the Faculty of Agriculture, Lorestan University, using a completely randomized design. Sodium selenate was applied at four concentrations (0, 0.5, 1, and 1.5 μM). Morphophysiological and biochemical parameters were assessed at the flowering stage. The results indicated that sodium selenate at a concentration of 0.5 μM significantly increased plant height, stem length, number of stems and leaves, and the fresh and dry weights of stems and aerial parts (flowers and leaves). Furthermore, sodium selenate at concentrations of 0.5 and 1 μM enhanced relative leaf water content, photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids, and total chlorophyll), chlorophyll a/b ratio, and chlorophyll stability index. However, these parameters declined at the higher concentration of 1.5 μM. The activities of antioxidant enzymes, including catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), also increased significantly following foliar application of selenium. Overall, the findings suggest that a low selenium concentration (0.5 μM) improves the growth and performance of summer savory by increasing photosynthetic pigments and strengthening the antioxidant defense system, whereas higher concentrations induce toxic effects and inhibit plant growth.