Plants possessing allelopathic potential could be used as a source of bio-herbicide to help decrease the use of synthetic herbicides. Acacia catechu (L.f.) Willd., a deciduous tree from the Mimosaceae family, has been reported to have medicinal properties. However, there have been no reports on the allelopathy of this tree. Therefore, the leaf extracts of A. catechu were examined for allelopathic potential using six concentrations: 0.001, 0.003, 0.01, 0.03, 0.1, and 0.3 g dry weight equivalent extract mL-1. The aqueous methanol extracts of A. catechu significantly suppressed the seedling growth of six test plants such as alfalfa, cress, lettuce, barnyard grass, Italian ryegrass, and timothy. The extracts inhibited the six test plants in a concentration-dependent manner. The shoot and root growth of all the test plant species were completely inhibited from the concentration of 0.3 g of dry weight equivalent extract mL-1, except the shoot growth of barnyard grass. Concentrations needed for 50% growth inhibition (I50 values) ranged from 0.004 to 0.043 g dry weight equivalent extract mL-1 for shoot growth, and 0.003 to 0.019 g dry weight equivalent extract mL-1 for root growth. Moreover, the I50 values indicated that the root growth of all the test plants was more susceptible to the A. catechu extracts than the shoot growth. The inhibitory effects of the extracts of A. catechu suggest that the extracts may contain allelopathic potential and, therefore, may be a potential candidate for the isolation and characterization of allelochemicals to develop an ecofriendly bio-herbicide.