We report a novel bioinspired underwater adhesive based on the injectable aqueous solution of a graft copolymer with a thermoresponsive backbone, which turns into a sticky hydrogel just below body temperature. With this topology, the collapse of the backbones upon the thermal transition leads to the formation of a percolating network of strong hydrophobic domains. Similar to Pressure-Sensitive Adhesives (PSAs), the hydrogel goes through fibrillation and extensive energy dissipation in large deformations, giving it an edge over conventional chemical hydrogels which are typically elastic and inherently non-sticky. This capability comes from the hydrophobic nano-scaffold which resists large deformations to minimize its contact with water. Since hydrophobic interactions are not weakened in water, the behavior of the hydrogel is maintained in aqueous medium. Chemistry-insensitive adhesion of this hydrogel offers a major advantage over current injectable adhesives which rely on in-situ chemical crosslinking reactions with tissues.