In the present article, we propose a fully coupled thermo-elasto-plastic-damage theory, which includes both kinematic and isotropic hardening, and also accounts for hydrogen diffusion in metals. This theory is based on the thermodynamics of irreversible processes under small deformation hypothesis and suppose that hydrogen diffuses in both normal interstitial lattice sites and trapping sites. This model is implemented into Abaqus/Standard by developing a UEL user subroutine and using an “assumed strain” method. A numerical application is performed to simulate hydrogen diffusion in a metallic welded joint submitted to a long-time severe environment at constant temperature.