Recently, highly stretchable and tough hydrogels that are photodegradable on‐demand have been reported. Unfortunately, the preparation procedure is complex due to the hydrophobic nature of the photocrosslinkers. Herein, a simple method is reported to prepare photodegradable double‐network (DN) hydrogels that exhibit high stretchability, toughness, and biocompatibility. Hydrophilic ortho‐nitrobenzyl (ONB) crosslinkers incorporating different poly(ethylene glycol) (PEG) backbones (600, 1000, and 2000 g mol−1) are synthesized. These photodegradable DN hydrogels are prepared by the irreversible crosslinking of chains by using such ONB crosslinkers, and the reversible ionic crosslinking between sodium alginate and divalent cations (Ca2+). Remarkable mechanical properties are obtained by combining ionic and covalent crosslinking and their synergistic effect, and by reducing the length of the PEG backbone. The rapid on‐demand degradation of these hydrogels is also demonstrated by using cytocompatible light wavelength (λ = 365 nm) that degrades the photosensitive ONB units. The authors have successfully used these hydrogels as skin‐worn sensors for monitoring human respiration and physical activities. A combination of excellent mechanical properties, facile fabrication, and on‐demand degradation holds promise for their application as the next generation of substrates or active sensors eco‐friendly for bioelectronics, biosensors, wearable computing, and stretchable electronics.