Reconfigurable intelligent surface (RIS)-assisted transmission and space shift keying (SSK) appear as promising candidates for future wireless communication systems. In this paper, a low-complexity cosine similarity-based method for phase optimization in RIS-assisted SSK systems with passive beamforming is proposed. Specifically, based on cosine similarity, we optimize the phase shift of reflected elements by maximizing the Euclidean distance (ED) between the different emission vectors corresponding to the ideal received signal, which shows a lower complexity compared with the sub-exhaustive search algorithm. Moreover, the proposed method avoids the poor bit error rate (BER) performance of the method based on maximizing the instantaneous receive signal-to-noise ratio (SNR). Our simulation results demonstrate the superiority of the proposed solutions over the relevant benchmarks.