Title: Provably efficient multi-task Reinforcement Learning in large state spaces. Abstract: We study multi-task Reinforcement Learning where shared knowledge among different environments is distilled to enable scalable generalization to a variety of problem instances. In the context of general function approximation, Markov Decision Process (MDP) with low Bilinear rank encapsulates a wide range of structural conditions that permit polynomial sample complexity in large state spaces, where the Bellman errors are related to bilinear forms of features with low intrinsic dimensions. To achieve multi-task learning in MDPs, we propose online representation learning algorithms to capture the shared features in the different task-specific bilinear forms. We show that in the presence of low-rank structures in the features of the bilinear forms, the algorithms benefit from sample complexity improvements compared to single-task learning. Therefore, we achieve the first sample efficient multi-task reinforcement learning algorithm with general function approximation.