In this paper, the effect of viscoelasticity of adhesives on shear stress distribution in the adhesive layer of a single-lap joint under shear load is studied. The joint comprises two elastic single isotropic adherend layers joined by a viscoelastic adhesive polymer. A three-parameter viscoelastic solid model is used to deduce the governing differential equation in the Laplace domain, which is solved using residue theorem. Results show that for an impulse load of 100N, the maximum shear stress in the adhesive layer is reduced to 39% of its initial value. Also, the ratio of viscous modulus of the adhesive to its shear modulus has an adverse effect on the peak shear stress. An increase in the thickness of the adhesive layer reduced the induced peak shear stress in the joint. Moreover, the shear strain in the adhesive layer reached its steady value after 1000 seconds.