Carrageenans (CGs) are hydrophilic, linear sulfated polysaccharides extracted from red algae. CGs have been utilized for tissue engineering due to their biocompatibility, non-toxic nature, and excellent physical properties. However, CGs form helical structures, which give them a gel-formation ability; hence, most CG hydrogels are thermoreversible and prepared using ion-crosslinking methods. Because these hydrogels cannot carry heat-sensitive drugs, growth factors, and cells, their in vivo application is limited. To solve these problems, an enzymatic crosslinking method to prepare CG hydrogels was introduced. Injectable hydrogels can seal wounds with minimal invasion and be crosslinked under mild conditions. In this study, an in-situ enzymatic crosslinking method based on horseradish peroxidase (HRP)-mediated reactions with hydrogen peroxide was utilized to synthesize CG-based hydrogels. We first prepared kappa-carrageenan poly (ethylene glycol)-tyramine (ka-CGPT) conjugates. Firstly, PNC-PEG-PNC was conjugated with tyramine (TA) via a urethane bond. Secondly, NH2 was conjugated with PNC-PEG-TA. Thirdly, oxidized carrageenan was conjugated with NH2-PEG-TA by EDC/NHS coupling chemistry. The CGPT conjugates were then used to prepare hydrogels via horseradish peroxidase (HRP)-mediated reactions in the presence of hydrogen peroxide. The gelation time was controlled from 5 sec to 5 min with various concentrations of HRP (0.1–0.5 mg/ml). It was found that the hydrogen peroxide concentration (0.3–0.7 wt%) can control the mechanical strength (150–700 Pa) of the hydrogels. In vitro 2D cell viability studies demonstrated that the ka-CGPT hydrogels had excellent bioactivity. Such modified injectable hydrogel platforms are promising materials and will be utilized for various tissue engineering strategies.