Enzymatic crosslinking of polymer–phenol conjugates in the presence of horseradish peroxidase (HRP) and H₂O₂ has emerged as an important method to synthesize in situ-forming, injectable hydrogels. Here we show that the alginate–dopamine (Alg–DA) conjugate, a polymer with catechol side groups instead of phenol groups, also gels in situ in the presence of HRP and H₂O₂. The effects of various factors, including the concentration of HRP, H₂O₂, and the polymer, and the degree of substitution of the polymer, on the gelation rate and the mechanical strength of the resulting gels, were studied by rheology. The influence of these factors on the gelling of catechol-functionalized polymer is similar to their influence on phenol-functionalized polymers, suggesting that they have a similar crosslinking mechanism. Compared to the phenol-functionalized alginate–tyramine (Alg–TA) hydrogel, catechol-functionalized Alg–DA gels exhibit significantly improved adhesion properties. When both the polymers have a degree of substitution of 10%, the adhesion strength of the latter is about 10-fold of the former. Replacement of phenol groups with catechol groups also results in very different cell behaviour. While the cells seeded on the Alg–TA gels do not attach onto the substratum, they attach onto the Alg–DA gels and exhibit a spread morphology. The significantly enhanced adhesion properties of the Alg–DA hydrogels are attributed to the catechol moiety, a structure found in the adhesive proteins of blue mussels.