Spiders and velvet worms produce strong yet sticky silks, often utilizing water for prey capture. Integrating mechanical strength and self-adhesion into artificial hydrogel fibers is equally important for smart capturing applications in miniature soft robotics. However, achieving both properties in a single material remains challenging due to the nearly contradictory network design requirements. Here, we present an ultrastrong, phase-separated copolymer hydrogel microfiber exhibiting water-activated self-adhesion. Continuously produced via pultrusion spinning, this microfiber features a hierarchical structure comprising nanoconfined hydrogen-bonded clusters embedded within a hygroscopic zwitterionic matrix. Under ambient conditions, the microfiber is strong and tough, with an ultrahigh Young’s modulus of 1.3 GPa and a fracture energy of 228.5 kJ m−2. Hydration disrupts the physical crosslinking, softening the fiber and releasing mobile dangling chains, which enables instant self-adhesion and results in a remarkable cured interfacial toughness of 1689 J m−2. This dynamically switchable, water-mediated adhesion establishes a new paradigm for designing high-performance adhesive fibrous materials.