AbstractIn this article, we propose a random quadrature spatial modulation (RQSM) scheme to achieve secure transmission in multiple‐input multiple‐output multiantenna eavesdropper channels. We consider the case in which the eavesdropper has more antennas than that of the legitimate receiver, leading to a strong wiretap channel. In this case, the conventional secure quadrature spatial modulation schemes based on the artificially interfering signals are vulnerable to eavesdropping. On the contrary, the proposed RQSM scheme is able to achieve the physical‐layer security without transmitting artificial noises. Specifically, the bits‐to‐symbol mapping of the proposed RQSM is randomized according to the instantaneous channel state information of the legitimate link. Since in a rich scattering environment, the legitimate and the wiretap channels are generally independent of each other due to the spatial decorrelation property. Thus, the eavesdropper can be considered ignorant of the legitimate channel and cannot decode the confidential messages correctly. We derive the ergodic secrecy rate of the proposed RQSM scheme and show that the secrecy performance does not degrade as the number of antennas at the eavesdropper increases, which also outperforms the conventional jamming strategies. In addition, the bit error rate of the RQSM is also analyzed, which shows that security is achieved without sacrificing the system's reliability. Numerical results validate the theoretical derivations and demonstrate the effectiveness of the proposed RQSM scheme.