Cognitive radio systems based on non-contiguous orthogonal frequency division multiplexing (NC-OFDM) is a promising technique that can provide efficient bandwidth utilization for high data rate wireless communication systems under frequency selective fading environments. Nevertheless, the sidelobe power of the secondary users (SU) in NC-OFDM modulated signals create out-of-band power (OBP) which induce interference to the primary users (PU) in the cognitive radio system. Moreover, OFDM has an inherent problem of high peak to average power ratio (PAPR). In this paper, we present a joint method for the PAPR reduction and sidelobe suppression based on partial transmit sequence (PTS) method. The objective is modelled as an optimization problem to minimize PAPR with a constraint on the maximum tolerable sidelobe power. The key idea is to generate some phase factors that meet the constraints of the proposed joint optimization problem. We examine the performance of the proposed joint method by computer simulation studies and the results show that it can significantly reduce PAPR and sidelobe power simultaneously. © 2015 IEEE.