Achieving strong and long-lasting underwater adhesion remains a great challenge because both interfacial hy-dration film and continuous water erosion significantly impede the interfacial bonding. Herein, a sequential self-assembly and self-coacervation strategy was proposed to actuate water-triggered robust underwater bonding to both abiotic materials and biological tissues. With the embedded polyhedral oligomeric silsesquioxane (POSS) cages serving as rigid and hydrophobic segments, the amphiphilic hybrid hyperbranched polymer featured conformation variation capacity under the synergy of sharply decreased solubility and cluster effect of poly-functional groups on POSS cages. The polymer firstly altered conformation into an assembled adhesive with extremely rigid hydrophobic POSS clusters for significantly enhancing the cohesion. Once touching water, the prompt aggregation of POSS clusters and followed self-coacervation behavior of the adhesive boosted interfacial water expulsion and catechol groups exposure, resulting in spontaneous solidification and universally eminent underwater adhesion between the adhesive and diverse substrates. The adhesive not only exhibited outstanding adhesive performances in water, seawater, PBS, acid, and alkali solutions, but also possessed excellent anti-erosion capacity to demonstrate long-lasting underwater adhesion for months. Prominently, a smart bio-adhesive was developed via introducing tissue adhesive group and sensitive disulfide bond, which revealed strong adhesion to wet tissues and on-demand debonding capacity. The water-triggered robust and long-lasting underwater bonding mediated by sequential self-assembly and self-coacervation opens up a new horizon in the design and progress of smart and high-performance underwater adhesives.
