Abstract <p>The influence of drought and exogenous melatonin on barley plants (<i>Hordeum vulgare</i> L.) of the Biоm variety was studied under soil experimental conditions. It was shown that soil drought reduced the biomass of barley plants, intensified the degradation of photosynthetic pigments, and led to oxidative stress in the plants, as evidenced by the increased levels of substances reactive to thiobarbituric acid. A single watering of barley plants with a melatonin solution increased both the fresh and dry biomass of the above-ground parts of the plants; under subsequent drought conditions, melatonin helped maintain plant biomass, reduced lipid peroxidation, sustained the levels of photosynthetic pigments, increased water content, and promoted an increase in the osmotic potential of the cell exudate. The study results suggest that exogenous melatonin protected plants from drought-induced oxidative stress by neutralizing free radicals, activating enzymatic (peroxidase) and non-enzymatic (carotenoids) antioxidant defenses, and regulating physiological processes (such as maintaining chlorophyll levels and water content), which contributed to enhanced drought resistance in the plants.</p>