It is a common opinion that main challenge for plant production is to cope with environmental stresses, both abiotic and biotic. Although biostimulator Asahi SL has been used for years to improve plant status and obtain possibly the highest and best quality yield, especially under conditions unfavourable for plant cultivation, its mode of action is still not understood. In this work the effect of Asahi SL on Arabidopsis thaliana plants grown under drought stress was studied. Plants grown under drought stress conditions and treated with Asahi SL were, as compared to untreated, higher and more advanced in development, particularly generative. Biomass accumulation was stimulated by biostimulator mainly due to better photosynthetic apparatus efficiency manifested by a higher (i) assimilation area, (ii) chlorophyll content, (iii) intensity of photosynthesis and (iv) improvement of chlorophyll a fluorescence parameters. Asahi SL treated plants were characterized by a lower concentration of biological active ABA. Despite the higher transpiration and lowered stomatal resistance RWC and water content were unchanged in Asahi SL treated plants which can be explained by increased water uptake in biostimulator sprayed plants. The obtained results clearly showed that the application of biostimulator played a protective role against drought stress.