It is well known that the high peak-to-average power ratio (PAPR) and large spectrum sidelobe power are two main drawbacks at the transmitter in noncontiguous orthogonal frequency-division multiplexing (NC-OFDM)-based cognitive radio (CR) systems. In this paper, we propose a novel signal cancelation (SC) method for joint PAPR reduction and sidelobe suppression in NC-OFDM-based CR systems. The key idea of the proposed method is to dynamically extend part of the constellation points on the secondary user (SU) subcarriers and add several SC symbols on the primary user (PU) subcarriers to generate the appropriate cancelation signal for joint PAPR reduction and sidelobe suppression. Then, the SC method formulates the problem of the joint PAPR reduction and sidelobe suppression as a quadratically constrained quadratic program (QCQP) to obtain its optimal cancelation signal. Moreover, we also propose a suboptimal SC (sub-SC) method to efficiently solve the QCQP optimization problem with low computational complexity. Simulation results show that the proposed SC method and the sub-SC method can provide both significant PAPR reduction and sidelobe suppression performances.