This paper presents a numerical and experimental study of residual stresses and distortions induced by the T-joint welding of two plates. Within the framework of numerical investigations, a thermo-mechanical finite element analysis is performed by using a shell/three-dimensional modeling technique to improve both the computational efficiency and the accuracy. The influence of the choice of the local 3D model size on the temperature, residual stress, and displacement distributions is investigated. A minimal 3D zone size that had both appropriate convergence of the solution and accuracy is defined. To validate the numerical model, a series of experiments using a fully automated welding process are conducted. A thermographic camera and an optical measurement system are used to measure the temperature and displacement distributions.