Plants during different phases of growth produce secondary metabolites which have multipurpose roles. One of the aspired roles associated with the production of such compounds is their active involvement in allelopathy-a phenomenon of the chemical interactions of plants and/or microorganisms. Investigation of the allelopathic properties of different plants is useful in agriculture as it would lead to the proper cultivation of crops under field conditions with respect to allelopathic plants. This study was aimed at exploring the allelopathic effects of Cassia fistula aqueous extracts obtained from leaves, bark, and fruit on germination and growth of Lactuca sativa, Setaria italica and Pennisetum americanum. Results demonstrated that aqueous extract from leaves, bark, and fruit of C. fistula had differential toxicity against the Lactuca sativa, Setaria italica and Pennisetum americanum. Phytotoxicity of extracts depended upon the concentration soaking duration. It was observed that 24-hour extracts were more inhibitory. Leaf extracts exhibited more toxicity than bark and fruit. Lactuca sativa was found more sensitive to the applied allelopathic stress of C. fistula than Setaria italica and Pennisetum americanum. Based on our results, it is suggested that Cassia fistula has strong allelopathic potential and may serve as a potential candidate for using its allelopathic potentials in weed control strategies.