This work theoretically investigates how an external magnetic field affects the resonant tunneling of terahertz radiation through a structured semiconductor film that is induced by the surface magnetoplasmons (SMPs). The applied external magnetic field with the Voigt configuration red shifts the frequencies of the resonant tunneling, by exciting the low-frequency branch of the SMP and reducing the effective plasma frequency. However, when the high-frequency-background relative permittivity of the semiconductor film markedly exceeds unity, the high-frequency branch of the SMP appears in the second forbidden band of the bulk magnetoplasmon. The semiconductor film has new resonant tunneling frequencies, which arise from the excitation of the high-frequency branch of the SMP. The applied external magnetic field blue shifts these new resonant tunneling frequencies. This phenomenon is attributed to the applied magnetic field increasing the effective plasma frequency.