Title: Factored Action Spaces in Deep Reinforcement Learning. Abstract: Very large action spaces constitute a critical challenge for deep Reinforcement Learning (RL) algorithms. An existing approach consists in splitting the action space into smaller components and choosing either independently or sequentially actions in each dimension. This approach led to astonishing results for the StarCraft and Dota 2 games, however it remains underexploited and understudied. In this paper, we name this approach Factored Actions Reinforcement Learning (FARL) and study both its theoretical impact and practical use. Notably, we provide a theoretical analysis of FARL on the Proximal Policy Optimization (PPO) and Soft Actor Critic (SAC) algorithms and evaluate these agents in different classes of problems. We show that FARL is a very versatile and efficient approach to combinatorial and continuous control problems.