Abstract-Non-cooperative identification of unknown communication signals is a popular research area with widespread civilian and military applications. Multiple Input Multiple Output (MIMO) systems employing multi-antenna transmission pose new challenges to the signal identification systems, such as the classification of the employed space time block code (STBC) and the modulation in presence of the self-interference inherent to the multi-antenna transmission. In the existing literature, these two classification problems have been handled separately, despite the fact that they are interrelated. This work presents a novel approach to MIMO signal identification by considering the modulation type and the STBC classification tasks as a joint classification problem.