Title: COPA: Certifying Robust Policies for Offline Reinforcement Learning against Poisoning Attacks. Abstract: As reinforcement learning (RL) has achieved near human-level performance in a variety of tasks, its robustness has raised great attention. While a vast body of research has explored test-time (evasion) attacks in RL and corresponding defenses, its robustness against training-time (poisoning) attacks remains largely unanswered. In this work, we focus on certifying the robustness of ofﬂine RL in the presence of poisoning attacks, where a subset of training trajectories could be arbitrarily manipulated. We propose the ﬁrst certiﬁcation framework, COPA, to certify the number of poisoning trajectories that can be tolerated regarding different certiﬁcation criteria. Given the complex structure of RL, we propose two certiﬁcation criteria: per-state action stability and cumulative reward bound. To further improve the certiﬁcation, we propose new partition and aggregation protocols to train robust policies. We further prove that some of the proposed certiﬁcation methods are theoretically tight and some are NP-Complete problems. We leverage COPA to certify three RL environments trained with different algorithms and conclude: (1) The proposed robust aggregation protocols such as temporal aggregation can signiﬁcantly improve the certiﬁcations; (2) Our certiﬁcations for both per-state action stability and cumulative reward bound are efﬁcient and tight; (3) The certiﬁcation for different training algorithms and environments are different, implying their intrinsic robustness properties. All experimental results are available at https://copa-leaderboard.github.io.