Controlled islanding is the last remedial action to prevent cascading outages or blackouts in power systems. Conventional methods presented for controlled islanding strategy determination, particularly those calculating load shedding values using optimization methods, are not fast enough in online applications for modern power systems. In this paper, a novel learning-based approach is introduced for online coherency-based controlled islanding in transmission systems. The proposed approach presents a prediction and optimization model, which is faster than conventional optimization-based models in two ways. Firstly, the proposed approach uses a classification model to predict the splitting scheme in a short time following the occurrence of a disturbance, and secondly in the proposed approach, a simpler optimization problem with fewer variables is solved to find the load shedding amount required in each area. In the proposed load shedding approach, some candidate system partitioning schemes are calculated beforehand and therefore, the load shedding optimization problem is simplified significantly compared to similar optimization-based approaches. Note that appropriate features, which are used in this paper as the input of the classifier, are acquired by processing post-disturbance phase angle variations, which are measured across the network. The proposed approach is simulated on the 16-machine, 68-bus system, and its accuracy and efficacy have been demonstrated.