The emerging human–machine interfacing applications have driven the developments of soft electronic devices that can seamlessly bridge electronics and tissues. Despite impressive progress, mismatches between current bioelectronic devices and biological tissues in both mechanical properties and charge transportation still exist, which could cause severe pain and damage to tissues. Herein, we present an ultra-stretchable all-hydrogel device with custom-designed microchannel patterns perfused with ionic liquids. Hydrophobic ionic liquids (ILs) are sufficiently conductive and remain stably separated from aqueous surroundings both in air and underwater, representing a perfect soft conductor for stretchable electronics. The potential of such designer IL-perfused all-hydrogel devices is demonstrated as soft conductors for triggering light-emitting diodes and as soft sensors for skin-conformed sensing under large stretches. We envision that such soft ionic devices may be useful for stimulating neurons and muscles in vivo with reduced immune responses.