Title: Exploration in Policy Mirror Descent. Abstract: Policy optimization is a core problem in reinforcement learning. In this paper, we investigate Reversed Entropy Policy Mirror Descent (REPMD), an on-line policy optimization strategy that improves exploration behavior while assuring monotonic progress in a principled objective. REPMD conducts a form of maximum entropy exploration within a mirror descent framework, but uses an alternative policy update with a reversed KL projection. This modified formulation bypasses undesirable mode seeking behavior and avoids premature convergence to sub-optimal policies, while still supporting strong theoretical properties such as guaranteed policy improvement. An experimental evaluation demonstrates that this approach significantly improves practical exploration and surpasses the empirical performance of state-of-the art policy optimization methods in a set of benchmark tasks.