In this paper, the dynamic response of an infinite periodic viaduct (IPV) subjected to an moving mass and harmonic seismic waves simultaneously is studied using Fourier transform method and finite element method (FEM). According to the periodicity of the IPV, the mass–viaduct contact force (MVCF) is expanded into Fourier series, each of which represents a moving load component of the MVCF. The response of the IPV to the moving load component can be obtained by using the response of the IPV to a frequency-wavenumber domain unit load determined by the FEM. The response of the IPV to harmonic seismic waves can also be determined via the FEM. With the obtained responses of the IPV to the moving load component and seismic waves as well as the equation of the motion for the moving mass, the coupling equation between the moving mass and the IPV is derived, with which the Fourier coefficients of the MVCF can be evaluated. Superposition of the response of the IPV to the moving load components and that to seismic waves yields the total response of the IPV to the moving mass and seismic waves. The presented numerical results indicate that for the simplified IPV model used in this study, the in-plane transverse displacement of the beam of the IPV due to the joint action of the moving mass and seismic wave is larger than that due to the seismic wave alone, while for the out-of-plane vibration, the transverse displacement of the beam of the IPV due to the moving mass and seismic wave together is usually smaller than that due to the seismic wave only.