Voice service is currently the main service in mobile satellite communications, but in the future, there will be a demand for high-data rate services offering multimedia contents. Therefore, an onboard processor capable of handling services of various data rates is required. In the present study, it is assumed that a non-regenerative onboard processor will be used. The onboard processor must have functions to filter signals and convert their frequencies in a channel-by-channel manner. An FFT filter bank satisfies these requirements. However, the conventional onboard FFT filter bank is only applicable to signals of constant bit rate. Hence, the onboard bandwidth-variable filter bank that can handle many signals of various bit rates is proposed. The principle of operation of the bandwidth-variable FFT filter bank is described, then the BER performance of an FPGA-implemented design is evaluated.