We investigated the solvated structure of cellulose in a phosphonate-based ionic liquid (IL) solution utilizing scattering experiments and all-atom molecular dynamics (MD) simulations. Based on the high-energy Xray total scattering experiment and MD simulations, a predominant interaction between cellulose and the IL was established, i.e., hydrogen bonding between the IL anion species and hydroxyl groups of cellulose. In addition, it was found that intramolecular hydrogen bonds existed within cellulose molecules, even when dissolved in the IL. Furthermore, the conformation of cellulose chains in the IL was investigated by a small-angle X-ray scattering experiment. As a result, it was found that cellulose molecules were dispersed at the molecular level and existed as rigid-rod-like polymers because of the intramolecular hydrogen bonds within the cellulose molecules. In dynamic light scattering experiments, a speckle pattern was observed for concentrated cellulose solutions. This indicated the existence of a physical-gel-like frozen inhomogeneity.