In this paper, we propose index modulated orthogonal frequency division multiplexing spread spectrum (IM-OFDMSS) scheme, which combines the techniques of SS and IM under the framework of OFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional M -ary modulated symbols. A low-complexity maximum ratio combining (MRC) detector is designed, in which the receiver first detects the selected spreading codes and then the corresponding data symbols are de-spread and demodulated. The bit error probability (BEP) upper bound and the achievable diversity order are analyzed according to the union bounding technique. Simulation results verify the analyses and show that the proposed IM-OFDM-SS outperforms the existing OFDM-IM and OFDM-SS schemes significantly.