Hydrogels were prepared by contacting air containing 10-50 ppm H2O2 with an aqueous solution containing polymer(s) possessing phenolic hydroxyl (Ph) moieties (polymer-Ph) and horseradish peroxidase (HRP). In this system, HRP catalyzes cross-linking of the Ph moieties by consuming H2O2 diffused from the air. The hydrogelation rate and mechanical properties of the resultant hydrogels can be tuned by controlling the H2O2 concentration in air, the exposure time of the air containing H2O2 to the solution containing polymer-Phs and HRP, and the HRP concentration. The shortest hydrogelation time of the solution stirred in air containing 16 ppm H2O2 was 6 s. Based on these findings, this hydrogelation system was applied to microextrusion bioprinting, in which bioink containing polymer-Phs, HRP, and cells were extruded into air containing H2O2. The superior cytocompatibility of the bioprinting method was confirmed by more than 90% viability, migration, and the spreading of mouse fibroblast 10T1/2 cells enclosed in the bioprinted hydrogels composed of derivatives of hyaluronic acid and gelatin, both possessing Ph moieties. These results demonstrate the great potency of HRP-catalyzed hydrogelation consuming H2O2 supplied in surrounding air for various biomedical applications, especially bioprinting.