In this letter, a simple product code is proposed for constant-amplitude biorthogonal multicode (CABM) modulation. In CABM modulation, vertical redundant bits are used for constant amplitude coding. The proposed product code can be constructed by using additional horizontal redundant bits. The hardware complexity of the encoder and decoder pair is very low. Simulation results show that the bit error rate performance of the system with the proposed coding scheme is improved as compared with conventional CABM demodulation.