Water deficit is one of the main environmental factors, and the recognition of plant response to drought stress seems to be crucial for the yield improvement of the important crops. The aim of the study was to investigate the effect of prolonged drought stress on dry matter yield, gas exchange parameters, and relative chlorophyll content in the leaves of Festulolium braunii and Trifolium repens grown in pure stands and in mixture. In the pot experiment, different levels of soil moisture (well-watered conditions and drought stress) and of cultivation method (pure stand and mixture) have been applied. The study has shown that all measured parameters were affected by drought stress. Dry mass yield, net photosynthetic rate (PN), transpiration rate (E), and stomatal conductance were significantly lower under drought stress than under well-watered conditions in all treatment types. T. repens showed the strongest response to stress, whereas F. braunii the weakest when both those species were grown in the pure stand. Under drought conditions, the yield of the mixture was similar to that of F. braunii cultivated in the pure stand, and significantly higher than that of T. repens cultivated in the pure stand. It was also found that under the stress, PN and E of both species in the the mixture was higher than in the pure stand. The highest water use efficiency was observed in F. braunii grown in the mixture. Our research shows, that in regions with limited rainfall, T. repens is more useful for growing in a mixture with F. braunii, than in the pure stand.