Rapid　and　strong　adhesion　of　hydrogel　adhesives　is　required　for　instant　wound　closure　and　hemostasis.　However,　in　situ　hydrogel　formation　and　sufficient　adhesion　at　target　tissue　sites　in　biological　environments　are　severely　compromised　by　the　presence　of　blood　and　body　fluids.　In　this　work,　an　underwater　adhesive　hydrogel　(named　SHCa)　is　fabricated　with　rapid　in　situ　gelation,　enhanced　mechanical　toughness,　and　robust　underwater　adhesion.　The　SHCa　can　undergo　rapid　UV　irradiation-induced　gelation　under　water　within　5　s　and　adhere　firmly　to　underwater　surfaces　for　6　months.　The　synergistic　effects　of　crystalline　β-sheet　structures　and　dynamic　energy-dissipating　mechanisms　enhance　the　mechanical　toughness　and　cohesion,　supporting　the　balance　between　adhesion　and　cohesion　in　wet　environments.　Importantly,　the　SHCa　can　achieve　rapid　in　situ　gelation　and　robust　underwater　adhesion　at　various　tissue　surfaces　in　highly　dynamic　fluid　environments,　substantially　outperforming　the　commercially　available　tissue　adhesives.　The　lap　shear　adhesion　strength　and　wound　closure　strength　of　SHCa　on　blood-covered　substrates　are　7.24　and　12.68　times　higher　than　those　of　cyanoacrylate　glue,　respectively.　Its　fast　hemostasis　and　wound　sealing　performance　are　further　demonstrated　in　in　vivo　animal　models.　The　proposed　hydrogel　with　strong　underwater　adhesion　provides　an　effective　tool　for　fast　wound　closure　and　hemostasis.　©　2022　American　Chemical　Society.