In this research, we explore for the first time the use of leaf stomatal conductance (gs) for phytotoxicity assessment. Plants respond to stress by regulating transpiration. Transpiration can be correlated with stomatal conductance when the water vapor pressure gradient for transpiration is constant. Thus, our working hypothesis was that the gs measurement could be a useful indicator of the effect of toxic compounds on plants. This lab-scale study aimed to test the measurement of gs as a phytotoxicity indicator. Our model plants were two common hydrophytes used in zero-effluent constructed wetlands for treating landfill leachate. The toxic influence of two types of leachate from old landfills (L1, L2) on common reed (Phragmites australis (Cav.) Trin. ex Steud.) and sweet flag (Acorus calamus L.) was tested. The gs measurements correlated well with plant response to treatments with six solutions (0 to 100%) of landfill leachate. Sweet flag showed higher tolerance to leachate solutions compared to common reed. The estimated lowest effective concentration (LOEC) causing the toxic effect values for these leachates were 3.94% of L1 and 5.76% of L2 in the case of reed, and 8.51% of L1 and 10.44% of L2 in the case of sweet flag. Leachate L1 was more toxic than L2. The leaf stomatal conductance measurement can be conducted in vivo and in the field. The proposed approach provides a useful parameter for indicating plant responses to the presence of toxic factors in the environment.