Title: Server Aggregation as Linear Regression: Reformulation for Federated Learning. Abstract: We propose a conceptually novel framework for Federated Learning (FL) called FedFit to mitigate issues of FL. FedFit is a reformulation of the server aggregation in FL, where the global model is updated by linear regression. This reformulation naturally enables us to utilize the established linear regression techniques for several FL issues. For example, we apply robust regression to alleviate the vulnerability issue against attacks on the global model from collapsed clients, and we apply LASSO regression to introduce sparsity into the model to reduce the communication cost in FL. Moreover, FedFit enables clients to upload compressed model parameters to the server, significantly reducing the data traffic. In experiments, we demonstrate that FedFit successfully improves robustness against attacks on a global model by robust regression and reduces the global model size by LASSO regression.