We synthesized a carboxymethylcellulose with phenol moieties by covalently incorporating tyramine into carboxymethylcellulose using aqueous-phase carbodiimide activation chemistry. The resulting hydrogel was obtained from an aqueous solution of the conjugate via the horseradish peroxidase-catalyzed oxidation reaction of phenols by Consuming H2O2, where the gelation speed depended on the concentrations of enzyme and H2O2. The viability of the mammalian cells enclosed within the hydrogel prepared from 1.5% (w/v) conjugate solution containing 5 units/ml horseradish peroxidase and 1 mM H2O2, was 80% after 24 h. These results demonstrate that this carboxymethylcellulose with phenol moieties has potential for biomedical applications including tissue-engineering.