This study presents the design, construction, and test results of an electromagnetic stress sensor for monitoring bridge cables and prestressed concrete structures. The sensor uses the reverse magnetostrictive effect found in high elastic limit steels such as those used in cables and in prestressed concrete. This effect is characterized by the variation in the steel's magnetic permeability as a function of its internal stress. Consequently, the internal stresses in this high elastic limit steels can be found by measuring their permeability. The permeability can be measured indirectly by measuring the inductance of a coil placed around or near the cable. We designed a prototype of the sensor with a finite element program. We also used this program to optimize the sensing coil and the measurement frequency and to design the magnetic shielding around the sensor. We built and tested the prototype in our laboratory. We evaluated the sensitivity, precision, linearity, and reliability of the sensor, and also the influence of external thermal and magnetic perturbations on the sensor measurements. The results were very satisfactory. The major advantages of this sensor are its robustness and its ability operate continuously for several decades even in hostile environments. These types of sensors, embedded in or added to the structure, are used to monitor stresses in cables and in prestressed concrete structures used in bridges and nuclear stations.