Title: SepRep-Net: Multi-source Free Domain Adaptation via Model Separation and Reparameterization. Abstract: We consider multi-source free domain adaptation, the problem of adapting multiple existing models to a new domain without accessing the source data. This is a practical problem, which often arises in commercial settings but remains an open question despite the advances in recent years. Previous methods, e.g., model ensemble, are effective, but they also incur significantly increased computational costs. Conventional solutions for efficiency, such as distillation, are limited in preserving source knowledge, i.e., maintaining generalizability. In this work, we propose a novel framework called SepRep-Net, which tackles multi-source free domain adaptation via model Separation and Reparameterization. Concretely, SepRep-Net reassembled multiple existing models to a unified network, while maintaining separate pathways (Separation). During training, separate pathways are optimized in parallel with the information exchange regularly performed via an additional feature merging unit. With our specific design, these pathways can be further reparameterized into a single one to facilitate inference (Reparameterization). SepRep-Net is characterized by 1) effectiveness: competitive performance on the target domain, 2) efficiency: low computational costs, and 3) generalizability: maintaining more source knowledge than existed solutions. As a general approach, SepRep-Net can be seamlessly plugged into various methods. Extensive experiments validate the performance of SepRep-Net on mainstream benchmarks.