This study was aimed at evaluating the effect of sublethal doses of glyphosate on physiological parameters of a common ornamental plant Mexican marigold (Tagetes erecta). The herbicide was applied in the following doses: 720 g ⋅ ha–1 (standard field dose), 144 g ⋅ ha–1, 28.8 g ⋅ ha–1, and 14.4 g ⋅ ha–1, in the form of a spraying treatment of plants in a specialist spraying chamber. The net assimilation rate and leaf greenness index were then determined. Herbicide application in the sublethal doses, i.e. below 720 g ⋅ ha–1, caused disorders in both analyzed physiological parameters of plants. The glyphosate dose of 144 g ⋅ ha–1 elicited transient disorders in the leaf greenness index. In turn, the use of the lower doses (28.8 g ⋅ ha–1 and 14.4 g ⋅ ha–1) caused a short-term increase in the net photosynthesis rate in the plants which was accompanied by a decreased value of the leaf greenness index. Study results demonstrated the effect of sublethal doses of glyphosate as a stress factor in parameters associated with the process of photosynthesis in plants.