A simple and efficient synthesis of pyrimido[b]quinolinetriones is introduced using copper (I) iodide nanoparticles (CuI NPs) as an efficient heterogeneous catalyst in green media-water. The catalyst was prepared through the simple precipitation route and well characterized by various techniques such as XRD and SEM. The mild reaction conditions, using aqueous medium, reusability of the catalyst, simple reaction work-up and excellent yields of products make the present protocol sustainable and advantageous compared to the conventional methods.