© 2022 by the authors.Phytomelatonin is a multifunctional molecule playing an important role in plant growth and development. In the present research, we studied the effects of exogenous melatonin (MEL) application on micro- and macro-morphological leaf traits (leaf area, stomatal index, stomatal size) of lemon balm (Melissa officinalis L.) plants. The study compared the application of MEL at four concentrations (0, 50, 100, and 150 mM). Increasing MEL concentrations up to a concentration of 100 mM induced positive effects on the examined micro/macro morphological traits (e.g., leaf mass area, relative leaf expansion rate, relative water content, relative stomatal area, stomatal pore index, the number of stomata, stomatal size), whereas these parameters were affected negatively when MEL was applied at a concentration of 150 mM. These effects of MEL application were associated to changes in stomata size and number. Our results provide evidences that MEL plays a key role in promoting growth in lemon balm and is a plant master regulator.