This study presents a novel structural modification strategy for lignin, utilizing a ternary eutectic solvent system (TESS), which induces targeted derivatization. The resulting lignin-based functional hydrogel (LBFH), prepared via rational cross-linking of derivatized lignin precursors, exhibits exceptional hygroscopic properties, with a water swelling ratio of 934.0%. Water absorption kinetics were subjected to rigorous analysis through the employment of a dual-modeling strategy that incorporates Schott kinetics and Fickian diffusion mechanisms, thereby elucidating the synergistic dynamic processes underlying surface adsorption and matrix penetration. Remarkably, LBFH maintains 48.6% water retention capacity after 7 days atmospheric exposure (25 °C, 60% RH), demonstrating unprecedented environmental stability among biopolymer hydrogels. The engineered properties of LBFH suggest its potential application in sustainable agricultural practices as drought-resistant soil amendments, and in environmental remediation as contaminant-adsorptive matrices.