Using outdated channel state information (CSI) in an opportunistic relaying system (ORS) leads to wrong selection of the best relay, which substantially deteriorates its performance. In this article, therefore, we propose a robust cooperative scheme coined opportunistic space-time coding (OSTC) to deal with the outdated CSI. A predefined number (i.e., N) of relays, instead of a single relay in ORS, are opportunistically selected from K cooperating relays. At the selected relays, N-dimensional orthogonal space-time coding is employed to encode the regenerated signals in a distributed manner. Then, N branches coded signals are simultaneously transmitted from the relays to the destination, followed by a simple maximum-likelihood decoding at the receiver. To evaluate its performance, the closedform expressions of outage probability and ergodic capacity are derived, together with an asymptotic analysis that clarifies the achievable diversity. Analytical and numerical results reveal that a full diversity of K is reaped by the proposed scheme when the knowledge of CSI is perfect. In the presence of outdated CSI, where the diversity of ORS degrades to one, the diversity of N can be kept. Moreover, OSTC’s capacity is remarkably higher than that of the existing schemes based on orthogonal transmission. From the perspective of multiplexing-diversity trade-off, the proposed scheme is the best cooperative solution until now. Index Terms—Alamouti, cooperative diversity, decode-andforward, generalized selection combining, opportunistic relaying, outdated channel state information (CSI), space-time coding.