This work investigates the secrecy outage performance of buffer-aided multi-relay multiple-input multiple-output (MIMO) cooperative systems in the presence of a passive eavesdropper. Due to the unavailability of the channel state information (CSI) of eavesdropper’s channel, a buffer-aided joint transmit antenna and relay selection (JTARS) scheme based on the main channel is proposed to enhance the secrecy performance. Specifically, we model the evolution of the relay buffers as a Markov chain and derive new exact and asymptotic closed-form expressions for the secrecy outage probability, which provides an efficient way to assess the effect of system parameters on the secrecy outage probability. Moreover, simple asymptotic results are further exploited under two special scenarios, i.e., L → ∞ and L → ∞ (where L denotes the size of the relay buffers), for characterizing the achievable secrecy diversity gain, the secrecy coding gain and the secrecy diversity-multiplexing tradeoff (DMT). Our results reveal that: 1), a secrecy diversity gain of NRMmin(NS,ND) is achieved when L → ∞, however, when L→∞, the secrecy diversity gain increases to NRM(NS +ND), where NS, NR and ND represent the number of antennas at the source, each of M relays and the destination, respectively. 2), The eavesdropper’s channel does not affect the secrecy diversity gain but only the secrecy coding gain in both two scenarios.