The fabrication of peptide assemblies to mimic the functions of natural proteins represents an intriguing aim in the fields of soft materials. Herein, we present a kind of novel peptide-based adhesive coacervate for the exploration of the environment-responsive underwater adhesion. Adhesive coacervates are designed and synthesized by self-assembled condensation of a tripeptide and polyoxometalates in aqueous solution. Rheological measurements demonstrate that the adhesive coacervates exhibit shear thinning behavior, which allows them to be conveniently delivered for interfacial spreading through a narrow gauge syringe without high pressure. The complex coacervates are susceptible to pH and metal ions, resulting in the occurrence of a phase transition from the fluid phase to the gel state. Scanning electron microscopy demonstrates that the microscale structures of the gel-like phases are composed of interconnected three-dimensional porous networks. The rheological study reveals that the gel-like assemblies exhibited mechanical stiffness and self-healing properties. Interestingly, the gel-like samples show the capacity to adhere to various wet solid substrates under the waterline. The adhesion strength of the peptide-based gel is quantified by lap shear mechanical analysis. The fluid coacervate is further exploited in the preparation of "on-site" injectable underwater adhesives triggered by environmental factors. This finding is exciting and serves to expand our capability for the fabrication of peptide-based underwater adhesives in a controllable way.