Building strong underwater bioadhesion is important for several applications but proved to be an extremely challenging task. In a wet/underwater environment, the water boundary layer hinders interfacial adhesion, which is further undermined by water-induced swelling in bioadhesives. In contrast to current hydrophilic bioadhesives, we here propose an underwater and in-situ applicable hydrophobic adhesive (UIHA). We report polydimethylsiloxane (PDMS) tangled with macromolecular silicone fluid as a dissipation phase to provide strong interface adhesion strength. The silicone fluid repels the surface boundary water layer, overcomes capillary break up in water, and rapidly gelates with PDMS, while a small amount of silane (