Title: Dual Ensembled Multiagent Q-Learning with Hypernet Regularizer. Abstract: Overestimation in the temporal-difference single-agent reinforcement learning has been widely studied, where the variance in value estimation causes overestimation of the maximal target value due to Jensen's inequality. Instead, overestimation in multiagent settings has received little attention though it can be even more severe. One kind of pioneer work extends ensemble methods from single-agent deep reinforcement learning to address the multiagent overestimation by discarding the large target values among the ensemble. However, its ability is limited by the ensemble diversity. Another kind of work softens the maximum operator in the Bellman equation to avoid large target values, but also leads to sub-optimal value functions. Unlike previous works, in this paper, we address the multiagent overestimation by analyzing its underlying causes in an estimation-optimization iteration manner. We show that the overestimation in multiagent value-mixing Q-learning not only comes from the overestimation of target Q-values but also accumulates in the online Q-network's optimization step. Therefore, first, we integrate the random ensemble and in-target minimization into the estimation of target Q-values to derive a lower update target. Second, we propose a novel hypernet regularizer on the learnable terms of the online global Q-network to further reduce overestimation. Experiments on various kinds of tasks demonstrate that the proposed method consistently addresses the overestimation problem while previous works fail.