We developed a novel thermo-reversible gelation system by mixing xyloglucan with an iodine solution. Xyloglucan dissolves in water to produce a colorless solution. However, the addition of iodine solution induces color and gelation; heating causes the gel to melt and become colorless. According to results from spectroscopic and small-angle X-ray scattering experiments, the gelation mechanism was proposed to involve two xyloglucan chains that associate side-by-side and incorporate iodine and/or iodide ions.