The use of space time block code (STBC) is analyzed in combination with spatial modulation (SM) for cooperative communications. A novel cooperation scenario and relay selection algorithm is proposed for wireless communications using cooperative spatial modulation. STBC is implemented in order to reduce the receiver complexity and achieve higher diversity gains. The proposed method is based on the selection of best relay from relay groups for cooperative STBC based SM. A low complexity maximum likelihood (ML) decoder is used to decide on the transmitted symbols and the relay indices. The bit error rates (BER) are calculated to test the transmission performance of the proposed method using computer simulations. Results show that the proposed STBC based cooperative SM scheme improves the bit error rate performance. Results demonstrate that our proposed method gives better performance than cooperative spatial modulation method proposed before, STBC-SM and STBC cooperative SM without relay selection.