Abstract: The vibration response of and sound radiation from an infinite fluid-loaded plate, stiffened by two dimensional periodically spaced structures and excited by a time-dependent plane harmonic pressure, are investigated in this paper. A semi-analytical approach based on the finite element method (FEM) and space harmonic method to study the stiffened plate is also presented. To obtain the reaction forces of the periodic structures acting on the plate, the FEM is applied by discretizing each structure into a sufficient number of elements and nodal points, and the reaction forces are approximated by the equivalent nodal forces. Then using the vibration equations of the periodic structures combined with the displacement boundary conditions between them and the plate, the nodal forces are expressed in terms of the corresponding discrete point displacements of the plate. Based on the space harmonic method and Fourier transforms, the vibro-acoustic equations of the stiffened plate are finally derived as functions of these point displacements of the plate, which are calculated numerically. In numerical examples, the validity of the present approach is demonstrated and the effects of the periodic structures on the vibro-acoustic responses of the plate are also analyzed.