The oxidative coupling of phenols by horseradish peroxidase (HRP) is widely utilized to cross-link polymer-phenol conjugates for hydrogel formation. Phenols containing one aromatic ring are most commonly used, and the addition of hydrogen peroxide (H2O2) is an indispensable step in catalyzing the enzymatic reaction. We describe here a hydrogel composed of polyphenol as the cross-linking moiety. (-)-Epigallocatechin-3-gallate (EGCG), a green tea catechin, was conjugated to hyaluronic acid (HA) to form HA-EGCG conjugates. Addition of HRP to a solution of HA-EGCG conjugates at pH 7.4 induced gelation in 7 min. Notably, the addition of exogenous H2O2 was not required, as H2O2 was generated via EGCG autoxidation. Moreover, cross-linking between HA-EGCG conjugates occurred in situ through EGCG quinone formation, even when no HRP was added. This approach of forming hydrogels circumvented the safety concern associated with HRP due to its plant origin. Furthermore, the EGCG moieties endowed the hydrogels with resistance toward hyaluronidase-mediated degradation in vivo.