This paper presents a novel method, ITMDSM, for the soil-foundation interaction problems analysis. It is a semi-numerical method based on the coupling of the Integral Transform Method (ITM) and the Dynamic Stiffness Method (DSM). The stiffness matrix of the soil-foundation system is obtained using the substructure technique. The ITM is used to obtain the solution of the wave propagation in the soil, while the DSM is used to calculate the dynamic stiffness matrix of the foundation. Both methods are operating in the frequency domain what makes them suitable for coupling. The number of numerical operations in the frequency domain is reduced by the application of the modal superposition technique. The analysis of vertical vibrations of flexible foundations resting on the viscoelastic half-space is presented. The formulation of the method could be generalized for horizontal and rocking vibrations. It could be also reduced to the problem of flexible strip foundations of infinite length.