The demand for vegetables was reinforced by the search for healthier foods, favoring increased sales of fresh products. Lettuce (Lactuca sativa L.) and tomato (Solanum lycopersicum L.) are the most cultivated vegetables in Brazil, thus being economically important. This study evaluates the allelopathic effect of aqueous extract of nutgrass (Cyperus rotundus) on lettuce (Lactuca sativa) and tomato (Solanum lycopersicum) plants, aiming to analyze its influence on species development. Two trials were conducted in the premises of the Taquaritinguense Institute of Higher Education (ITES) in Taquaritinga city, São Paulo State, under temperatures ranging from 14 ºC to 28 ºC, in a greenhouse. The variables analyzed were: fresh shoot and root weight (g), using a semianalytical scale; and root length (cm), using a ruler. Cyperus rotundus leaf extract at higher concentrations positively affected the early growth of vegetables. Studies on allelopathic effects between plants are extremely important because the use of this extract can contribute to seedling growth. The 200% concentration of Cyperus rotundus extract increased root and shoot development in tomato crop.