The present study describes enzymatically cross-linked gelatin-based hydrogels as in situ forming tissue adhesives. A series of gelatin derivatives with different phenolic contents were synthesized by conjugating hydroxyphenyl propionic acid and tyramine to gelatin backbones. Two gelatin derivatives, gelatin-hydroxyphenyl propionic acid (GH) and gelatin-hydroxyphenyl propionic acid-tyramine (GHT) with maximum obtainable phenolic contents (146.6 μmol g-1 GH and 395.7 μmol g-1 GHT), were used to prepare gelatin-based hydrogels via horseradish peroxidase (HRP)-mediated reactions in the presence of hydrogen peroxide (H2O2). By changing the HRP and H2O2 concentrations, the gelation time, mechanical strength, and degradation rate of the hydrogels were fairly well controlled, indicating a tunable rate and degree of cross-linking. In addition, we found that an increase in phenolic content led to increased mechanical strength of the hydrogels. Lap-shear test results clearly showed that the GH and GHT hydrogels exhibited 2-3 times greater tissue adhesiveness compared to fibrin glues. On the basis of these results, we conclude that in situ forming gelatin-based hydrogels, which are both injectable and sprayable, can be used as an alternative to conventional tissue adhesives.