This paper presents the ultimate shear strength of longitudinally stiffened steel-concrete composite plate girders (LS-CPGs) under shear loading. The tension field action in the girder web, shear capacity of concrete slab, and the position of longitudinal stiffeners are considered in driving an analytical method to predict the ultimate shear strength of the girder. The accuracy of the proposed method is evaluated by finite-element (FE) models. A three-dimensional FE model has been generated, considering both the geometric and material nonlinearity of LS-CPGs. The results show that longitudinal stiffeners increase the ultimate shear strength of LS-CPG. The comparison between the analytical and corresponding FE results shows that the proposed analytical method can accurately predict the ultimate shear strength of LS-CPGs.