There is a great demand for the fabrication of soft electronics using hydrogels due to their biomimetic structures and good flexibility. However, conventional hydrogels have poor mechanical properties, which restricts their applications as stretchable sensors. Herein, a facile one-step strategy is proposed to fabricate tough hydrogels with a semi-interpenetrating network structure by free-radical polymerization. The obtained polyacrylamide/carboxymethyl chitosan composite hydrogels possess outstanding transmittance and excellent mechanical performances, with tensile breaking stress of 260 kPa, breaking strain of 3300%, and toughness of 2400 kJ/m3. These hydrogels have low modulus of ~10 kPa, fast recoverability after unloading, and high conductivity of ~0.85 S/m without the addition of other conductive substances. The ionic conductivity of the gels originates from the counterions of carboxymethyl chitosan, affording the hydrogels as resistive-type sensors. The resultant hydrogel sensors demonstrate a broad strain window, excellent linear response, high sensitivity with a gauge factor, and great durability, capable of monitoring diverse human motions. This work provides a new strategy to develop stretchable conductive hydrogels with promising applications in the fields of artificial intelligence and flexible electronics.