This paper deals with the hygrothermal stress singularity induced at the interface corner between the epoxy coating layer and the concrete substrate as the coating layer absorbs hot moisture from the ambient environment. The epoxy layer is assumed to be a linear viscoelastic material and to be theromorheologically simple. It is further assumed that moisture effects are analogous to thermal effects. The viscoelastic boundary element method is employed to investigate the behavior of interface stresses. The order of the singularity is obtained numerically for a given viscoelastic model. The numerical results exhibit the relaxation of interface stresses and large stress gradients are observed in the vicinity of the free surface. Since the exceedingly large stresses cannot be borne by the epoxy coating layer, local yielding or the delamination can occur at the interface corner.