In this letter, a novel phase offset selected mapping (SLM) scheme is proposed to reduce the peak-to-average power ratio (PAPR) in Alamouti coded multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems, and its key idea is that different phase rotation sequences are multiplied by their corresponding phase offsets at the transmitter. Moreover, a minimum Euclidian distance (MED) decoder at the receiver is proposed to recover the phase rotation sequences. Therefore, the proposed SLM scheme does not need to reserve bits for the transmission of side information, resulting in the increase of the data rate. Theoretical analysis and simulation results show that the proposed SLM scheme could offer good performances of both the bit error rate and PAPR reduction.