A new straightforward method for residual stress measurement is presented. A specimen is cut in half along its diagonal by using electric discharge machining, and the deformation benchmarks are measured to determine the displacements caused by release of the residual stresses. Analytically, to verify calculation accuracy of the new method, the known stresses are applied in finite element model as an initial condition. By using the series expansion approach, the inverse calculation based on displacements gives the original longitudinal residual stress of the specimen. This “wedge method” is more straightforward than other relaxation methods because it measures fewer data. Its data processing is simpler, yet its result is found to be in good agreement with other methods. The new method is first outlined for a three-dimensional (3-D) body and the accuracy of the methodology is demonstrated for a representative case using a numerical experiment. Then measurements are performed on a block specimen of AA6061-T6 to show that the approach is valid under real experimental conditions.