The aim of this study was to determine the influence of water stress i.e. different irrigation norms on the allelopathic potential of lettuce (Lactuca sativa L.). The experiment was conducted in Petri dishes, and the allelopathic potential of water extracts prepared from lettuce grown at three different irrigation norms (45 mm, 90 mm and 135 mm) was tested in three concentrations (2.5%, 5% and 10%) on seed germination and seedling growth of radish. Allelopathic effect of water extracts was evaluated on the following parameters: seed germination, root and shoot length and fresh mass of radish seedlings. The results showed a statistically significant negative allelopathic potential of lettuce water extracts on seed germination and seedling growth of radish. The allelopathic potential depended on the concentration of water extracts and the irrigation norms i.e. the effect of water stress on lettuce. By increasing the concentration of water extracts, the negative allelopathic effect also increased, so that all lettuce extracts with a concentration of 10% showed a complete inhibitory effect (100%) on all measured parameters. Also, the influence of water stress and irrigation norms on the allelopathic potential of lettuce was determined. The influence of the irrigation norms on the allelopathic potential of lettuce was most visible when using water extracts in 5% concentration, since water extracts prepared from lettuce grown at two lower irrigation norms of 45 mm and 90 mm had a statistically significantly stronger inhibitory effect than extracts from lettuce grown at irrigation norm of 135 mm on all measured parameters.