Title: MEDOE: A Multi-Expert Decoder and Output Ensemble Framework for Long-tailed Semantic Segmentation. Abstract: Long-tailed distribution of semantic categories, which has been often ignored in conventional methods, causes unsatisfactory performance in semantic segmentation on tail categories. In this paper, we focus on the problem of long-tailed semantic segmentation. Although some long-tailed recognition methods (e.g., re-sampling/re-weighting) have been proposed in other problems, they are likely to compromise crucial contextual information in semantic segmentation. Therefore, these methods are hardly adaptable to the problem of long-tailed semantic segmentation. To address this problem, we propose a novel method, named MEDOE, by ensembling and grouping contextual information. Specifically, our MEDOE is a two-sage framework comprising a multi-expert decoder (MED) and a multi-expert output ensemble (MOE). The MED includes several ``experts", each of which takes as input the dataset masked according to the specific categories based on frequency distribution and generates contextual information self-adaptively for classification. The MOE then ensembles the experts' outputs with learnable decision weights. As a model-agnostic framework, MEDOE can be flexibly and efficiently coupled with various popular deep neural networks (e.g., Deeplabv3+, OCRNet, and PSPNet) to improve the performance in long-tailed semantic segmentation. Experimental results show that the proposed framework outperforms the current methods on both Cityscapes and ADE20K datasets by up to 2\% in mIoU and 6\% in mAcc.