Triboelectric nanogenerators (TENGs) are promising power sources for wearable devices, but simultaneously achieving high stretchability and electrical performance remains challenging. Here, we develop a highly deformable TENG featuring stretchable electrodes based on double-network ionic hydrogels and an elastomeric triboelectric layer incorporating high-dielectric nanoparticles. The double-network structure enhances mechanical durability while maintaining electrical functionality under large deformation. The device operates up to 400% strain, delivering an output voltage of ~100 V at 100% strain and a power density of 2.15 W/m². The TENG effectively monitors human motion, demonstrating its potential for self-powered wearable and healthcare applications.