The electrical steel used in motor cores frequently experiences magnetic property degradation owing to elastic deformation and residual stress introduced during manufacturing processes. The quantitative characterization of the inverse magnetostrictive behavior, however, remains challenging because conventional measurement approaches typically require bulky loading systems and laminated specimens. We present a compact measurement system for characterizing the inverse magnetostrictive behavior of an electrical steel sheet using a plane-stress needle-probe method. By applying the plane-stress elasticity principle, compressive strain can be generated through Poisson’s effect during tensile loading, enabling both tensile and equivalent compressive magnetic characterizations using a single-sheet specimen. A dual-core excitation structure is employed to ensure symmetric magnetic flux distribution across the sheet thickness, while the calibration based on Epstein-frame reference data provides quantitative consistency with standard magnetic measurements. Experimental evaluation on a 0.5-mm-thick non–grain-oriented electrical steel sheet demonstrates that compressive deformation causes a significantly greater magnetic degradation than tensile deformation, particularly in the low-flux permeability region. Comparative measurements on materials with different iron-loss grades further reveal variations in stress sensitivity that are relevant to motor design. The proposed measurement system provides a compact and practical platform for investigating stress-induced magnetic degradation in electrical steel and offers useful reference data for motor design and material development.