Fiber strain sensors with robust sensing performance are indispensable for human–machine interactions in the electronic textiles. However, current fiber strain sensors are confronted with the challenges of unavoidable deterioration of functional sensing components during wearable and extreme environments, resulting in unsatisfactory stability and durability. Here, we present a robust fiber strain sensor based on the mutual inductance effect. The sensor is assembled by designing coaxial helical coils around an elastic polyurethane fiber. When stretching the fiber sensor, the strain is detected by recording the voltage changes in the helical coils due to the variation in magnetic flux. The resultant fiber strain sensor shows high linearity (with a linear regression coefficient of 0.99) at a large strain of 100%, and can withstand various extreme environmental conditions, such as high/low temperatures (from - 30 °C to 160 °C), and severe deformations, such as twisting and pressing (with a pressure of 500 N/cm). The long-term cyclic stability of our fiber strain sensor (100,000 cycles at a strain of 100%) is superior to that of most reported flexible resistive and capacitive strain sensors. Finally, the mass-produced fiber strain sensors are woven into a smart textile system to accurately capture gestures.