Need for fresh water is ever increasing due to water pollution and desertification. Purification of seawater or wastewater becomes an essential method to obtain freshwater. Among various water treatment methods, forward osmosis (FO) is actively investigated due to its many advantages. In an FO system, a concentrated draw solution draws water from a feed solution by the osmotic pressure gradient. After the drawing, the solute in the draw solution was separated to give freshwater. Efficient draw solutes should have high osmotic pressure as well as easy separability. We thought that temperature-responsive smart materials, which can be easily separated after FO by controlling the temperature, are excellent candidates for draw solutes. It has been demonstrated that halide salts of branched polyethylenimine (b-PEI) show upper critical solution temperature (UCST) phase transition in aqueous solution. Inspired by the structures of UCST-type b-PEI halide salts, we prepared ammonium iodide salts with a similar structural moiety but with lower molar weight. These ammonium iodide salts had high osmotic pressure due to greatly reduced molecular weights. Lengths of carbon chains, ionic interactions, and symmetry of the salt structures were controlled to investigate their solubility, temperature and concentration range of phase transition, osmolality change, and stability. Among various ammonium iodide salts synthesized, HM8I and HM10I were chosen as potential draw solutes due to the appropriate range of transition temperature and solubility. Through the FO experiments, HM8I and HM10I have shown high water fluxes and negligible reverse solute fluxes compared to the conventional draw solutes. The structure of temperature-responsive ammonium iodide salts will provide a potential platform of practical draw solutes for future efficient and eco-friendly water treatment.