On-skin power sources require ultrathin form factors, safety, and mechanical robustness, but few battery platforms simultaneously satisfy these requirements. Here, we report the thinnest Al–air batteries to date (<270 µm) enabled by hygroscopic hydrogel electrolytes. Operating in a water-in-salt regime with highly concentrated ionic species, strong ion–water interactions suppress electrolyte dehydration and Al self-corrosion, while enabling high-capacity operation of 2479 mAh g‒1 at 10 mA cm‒2 in ultrathin architectures. The cells withstand over 3000 bending cycles at a bending radius of 1 mm and remain functional under severe deformation, including crumpling, cutting, and perforation. Stable operation over more than 1000 cycles of repeated anode replacement suggests an operational lifespan exceeding 8000 h. When integrated with organic electrochemical transistor-based biosensors, the system enables continuous, real-time physiological signal monitoring with a signal fidelity of up to 31.1 dB, providing a viable route toward continuous operation of conformal epidermal electronic systems.