In this letter, a reconfigurable intelligent surface assisted space-time spatial modulation for symbiotic radio (RIS-STSM-SR) is proposed. In this scheme, RIS adjusts its reflection coefficient based on channel state information and transmits additional information bits for Internet of Things (IoT) applications. Space-time codewords and a joint constellation modulation for the transmitter and RIS are designed. Thus, the proposed scheme provides high spectral efficiency and achieves the transmit diversity order of two. Furthermore, a low-complexity decoder (LCD) is proposed to reduce the complexity of ML decoder. Performance analysis and simulation results show that RIS-STSM-SR outperforms some of the existing RIS-IM schemes.