We have developed an injectable hydrogel system composed of biodegradable polymer–phenol conjugates that are covalently crosslinked by an enzyme-mediated oxidative reaction. The oxidative reaction of hydrogen peroxide (H2O2) and horseradish peroxidase (HRP) catalyzed the crosslinking of the phenol moieties and allowed the independent tuning of the mechanical strength and gelation rate. This injectable hydrogel system with an independent tuning property has great potential for controlled drug delivery as well as three dimensional (3D) cell culture and differentiation in the hydrogel.