SiYuan is an open-source, privacy-focused personal knowledge management and note-taking application. It uses block-based editing to organize notes, documents, references, databases, and linked knowledge in a flexible workspace, supporting local storage and self-hosted synchronization options. Users can build interconnected personal knowledge bases rather than keeping information in isolated documents.

The platform includes rich Markdown-style editing, backlinks, queries, templates, document relationships, data views, and extensibility through plugins and APIs. It is primarily a productivity and knowledge-management tool rather than an AI application, although its structured content and integrations can be useful alongside AI assistants and retrieval workflows. SiYuan is especially attractive to users who want a powerful alternative to cloud-first note systems while retaining control over their data.