Stable and reversible adhesion to wet surfaces is challenging owing to water molecules at the contact interface. In this study, we develop a hydrogel-based wet adhesive, which can exhibit strong and reversible adhesion to wet and underwater surfaces as well as to dry surfaces. The remarkable wet adhesion of the hydrogel adhesive is realized based on a synergetic integration of bioinspired microarchitectures and water-friendly and water-absorbing properties of the polymeric hydrogel. Under dry conditions, the microstructured hydrogel adhesive exhibits strong van der Waals interaction-based adhesion, while under underwater conditions, it can maximize capillary adhesion. Consequently, the hydrogel adhesive exhibits remarkable adhesion strengths for dry, moist, and submerged substrates. Maximum normal and shear adhesion strengths of 423 and 384, 492 and 340, and 253 and 21 kPa are achieved with the hydrogel adhesive for dry, moist, and submerged substrates, respectively. Our results demonstrate that strong wet and underwater adhesion can be achieved only with the hydrogel-based adhesive with simple microscale architecture.