Tributylmethylphosphonium iodide is primarily used as a phase-transfer catalyst in organic synthesis. It facilitates the transfer of reactants between immiscible phases, enhancing reaction rates and yields in processes such as alkylation, esterification, and polymerization. Its application is particularly valuable in the production of pharmaceuticals, agrochemicals, and specialty chemicals, where precise control over reaction conditions is crucial. Additionally, it is employed in the synthesis of ionic liquids, which are utilized in green chemistry for their low volatility and recyclability. The compound also finds use in electrochemical applications, including batteries and sensors, due to its ionic conductivity and stability.