Based on the theory of acoustoelasticity, a new ultrasonic stress reconstruction method—the generalized acoustic ratio (GAR) technique—is developed for locally plane structures and orthotropic materials. For given transit times of the three wave modes and the shear wave polarization angle, the local plane stress tensor is uniquely determined. The GAR technique yields accurate stress estimates with relatively small temperature sensitivity. Based on calibration constants from three uniaxial specimens, the entire stress field in a compact tension specimen is reconstructed. The results are in very good agreement with stress predictions from an elastoplastic finite element analysis. To further improve the measurements, a numerical technique, the stress field approximation (SFA) technique, is developed. The SFA technique uses a smooth local bicubic spline approximation and aims at improving the overall stress field estimate by enforcing the equilibrium equations, the stress boundary conditions and symmetry conditions. Numerical results show that both the average error and its spread are indeed reduced.