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Flexible strain sensors have enormous potential in wearable devices, robots, and health monitoring equipment. However, the poor stretchability of strain sensors based on semiconductors and the low sensitivity of resistance change‐based hydrogel strain sensors hinders the comprehensive application. Herein, a flexible piezoionic SnSe‐hydrogel composite with an optimized structure and improved performance is designed. The piezoionic output rises nonlinearly as the applied force increases, with the piezoionic coefficient up to 1780 nV Pa−1 and −7.21 nA Pa−1. The composite can realize the continuous positioning in 1D space based on the piezoionic effect. It also demonstrates self‐powered characteristics, an ultrafast response speed of 6–8 ms, and a high gauge factor of 95.89. The sensor is exemplified to monitor fist clenching and finger bending, which has the potential to discriminate different joint movements. Meanwhile, the device can light up a light–emitting diode under pressure and bending. The as‐prepared piezoionic SnSe‐hydrogel device, having both high stretchability and sensitivity, may shed light on developing high‐performance flexible strain sensors and generators. [ABSTRACT FROM AUTHOR]