Hydrogels were obtained from aqueous solution containing polymer(s) possessing phenolic hydroxyl moieties through horseradish peroxidase (HRP)-catalyzed reaction without direct addition of H2O2. In this hydrogelation process, H2O2 was generated from HRP and glucose contained in the aqueous solution, that is, HRP functioned not only as a catalyst, but also as a source of H2O2. The gelation time and mechanical properties of the resultant hydrogel could be altered by changing the concentrations of HRP and glucose. Cytocompatibility of the hydrogelation process was confirmed from cell studies using mouse 10T1/2 fibroblast cells.