<p>Industrial seawater electrolysis remains constrained in achieving both high catalytic activity and long-term durability, with key limitations including structural degradation and mechanical instability within catalyst layers. Here we show a self-adhesive high-entropy oxide sub-nanowire monolithic catalyst that overcomes both obstacles. The catalyst is synthesized under mild conditions and incorporates 14 metal elements into uniform ~1.2 nm sub-nanowires with strong intrinsic adhesion to conductive substrates, eliminating the need for external binders. It also features unconventional active sites that enable efficient and durable lattice oxygen activation while preserving structural integrity during prolonged operation. It exhibits overpotentials of 129 mV in 1 M KOH and 153 mV in 1 M KOH + seawater at 10 mA cm⁻², and maintains continuous operation at 1,000 mA cm⁻² for 4,700 h and 4,400 h, respectively. Integrated into an anion exchange membrane seawater electrolyser, it delivers 3,000 mA cm⁻² at 1.70 V (80 °C) and operates continuously for over 3,819 h at 2,000 mA cm⁻² under ambient conditions.</p>