Documenting architectural assumptions effectively and systematically is of paramount importance in software development. However, the lack of tool support is a critical problem when practitioners manage assumptions in their daily work. To fill this gap, we propose Architectural Assumptions Manager (ArAM) – a tool dedicated to Architectural Assumption Documentation, which aims at alleviating the aforementioned problem. ArAM was developed as a plugin of Enterprise Architect and provides integration with Unified Modeling Language (UML) modeling. ArAM was evaluated in terms of its perceived usefulness and ease of use, through a case study with 16 architects from ten companies in China. The results indicate that ArAM is generally useful and easy to use, both in Architectural Assumption Documentation and in software development in general. Moreover, there are several points for improvement, including support for automated analysis (e.g., finding missing relationships between assumptions) and verification (e.g., verifying the correctness of existing assumptions).