The secrecy outage performance of transmit antenna selection (TAS)/maximal ratio combining (MRC) in multiple-input–multiple-output (MIMO) cognitive radio networks (CRNs) over Rayleigh fading channels. In the considered system, a secondary user (SU-TX) equipped with NA ( N A ≥ 1) antennas uses TAS to transmit confidential messages to another secondary user (SU-RX), which is equipped with NB ( N B ≥ 1) antennas and adopts the MRC scheme to process multiple received signals. Meanwhile, an eavesdropper equipped with NE ( NE ≥ 1) antennas also adopts the MRC scheme to overhear the transmitted information between SU-TX and SU-RX. SU-TX adopts the underlay strategy to guarantee the quality of service (QoS) of the primary user (PU) without spectrum sensing. In this paper, we derive the exact and asymptotic closed-form expressions for the secrecy outage probability (SOP). Simulations are conducted to validate the accuracy of the analysis.