Online Sequence Diagram Editor — with Mermaid & PlantUML Support
SeqDiagram is a free online sequence diagram editor: type your flow in plain text and the diagram renders live beside you. No sign up, no install, nothing to configure. Import a Mermaid or PlantUML diagram to edit it visually and export back to either format. New to the notation? Skim the syntax guide, or open a ready-made example and edit it. When you're done, share it with a link or export to PNG, SVG, Mermaid, or PlantUML.
Everything you need, nothing you don't
Type it, see it instantly
Write your flow in plain text. The diagram updates live as you type. No drag and drop, no shapes to place — just write.
Full UML support
Participants, actors, synchronous and async messages, activation bars, notes, loops, alt/else, opt, and group fragments.
Paste straight into Slack or Notion
Click Copy Image and paste directly into Slack, Notion, Google Docs, or a PR comment. No saving files, no uploading.
Export PNG, SVG, Mermaid & PlantUML
Download PNG or SVG, or export to Mermaid (.mmd) for GitHub/GitLab/Notion or PlantUML (.puml) for Confluence and JetBrains IDEs.
Import Mermaid & PlantUML
Already have a diagram in Mermaid or PlantUML? Paste it in and the editor converts it automatically — edit visually, export to any format.
Share with one link
Hit Share and send the URL. The entire diagram is encoded in the link — no backend, no login needed for the person you send it to.
Zoom & pan
Navigate large diagrams with mouse wheel zoom and click-drag panning. Fit to screen in one click.
Free, no sign up ever
Open the page and start. No account, no subscription, no credit card. Your diagrams auto-save in your browser and stay private.
How to make a sequence diagram in under a minute
Declare your participants. Use participant Browser or actor User to define who is involved. The order you declare them sets the left-to-right column order.
Add messages between them. Write Browser->Server: GET /api for a solid arrow, or Server-->Browser: 200 OK for a dashed return. Use ->> and -->> for open arrowheads.
Add structure with fragments. Wrap steps in loop, alt/else, opt, or group blocks to show conditional and repeating logic.
Share or export. Hit Share to copy a link — anyone with it sees your diagram instantly, no login. Or download as PNG, SVG, or Mermaid, or paste straight to Slack with Copy Image.
What people use it for
Developers, architects, and tech leads use SeqDiagram to quickly sketch and share flows without setting up a tool or opening a heavy editor:
Start from a real example
The fastest way in is to open a worked example, see how it's written, and change it to fit your own flow. Each one opens straight in the editor:
See all of them on the examples page, or read the full syntax guide.
Frequently asked questions
What is a sequence diagram?
A sequence diagram is a type of UML (Unified Modeling Language) diagram that shows how a set of participants — users, services, databases, or any system component — communicate with each other over time. It captures the order of messages exchanged, making it one of the most widely used diagrams for documenting software behaviour.
Can I generate a sequence diagram from text?
Yes — that's the whole idea. You write the flow as plain text and the tool generates the diagram from it as you type. There's nothing to drag or place: describe who talks to whom, and the diagram is drawn for you. It works as a sequence diagram generator and a hand-editable editor at the same time.
Can I create a sequence diagram online for free?
Yes, genuinely free. No account, no trial period, no subscription. You can create as many sequence diagrams online as you like — it all runs in your browser, so there's nothing to charge you for.
Do I need to install anything?
Nothing at all. Open the page, start typing. Works in Chrome, Firefox, Safari, and Edge. No extensions, no plugins, no Java.
What syntax does this tool use?
The tool uses a human-readable plain-text syntax. Declare participants, write messages using arrow notation (->, -->, ->>, -->>), and wrap blocks in fragments like loop, alt, opt, and group. Click the Help button for a full reference with examples.
Can I use this for UML diagrams?
Yes. The tool supports the core UML sequence diagram notation: participants, actors, synchronous and asynchronous messages, activation lifelines, combined fragments, and notes.
How do I export my sequence diagram?
Use the PNG button in the toolbar to download a high-resolution raster image, or the SVG button for a scalable vector image. Both export immediately without any account required.
How do I share a diagram?
Click Share. You get a URL with the full diagram encoded in it. Send it to anyone — they open it in their browser and see the exact same diagram. No login needed, no account required on their end.
Can I use it for API and architecture docs?
Yes. Engineers use it for API design, microservice flows, auth sequences, and system integration specs. The plain-text format also works well in git repos — commit your .seq file alongside the code it documents.
Can I export a sequence diagram to Mermaid?
Yes. Click the Export Mermaid button to download a .mmd file in Mermaid syntax. This is compatible with GitHub markdown, GitLab, Notion, Obsidian, and any other tool that supports Mermaid sequence diagrams. You can also paste an existing Mermaid sequenceDiagram block into the editor and it converts automatically.
Can I import a PlantUML diagram?
Yes. Use the Import PlantUML option in the toolbar. Paste your @startuml / @enduml block and the editor converts it to the native syntax automatically so you can edit it visually and export to any format.
Can I export a sequence diagram to PlantUML format?
Yes. Use the Export PlantUML option to download a .puml file. The output is compatible with Confluence (via the PlantUML macro), JetBrains IDEs, and any other tool that renders PlantUML sequence diagrams.
How do I copy a sequence diagram to paste into Slack or Notion?
Click Copy Image in the toolbar. The diagram is copied as a PNG image to your clipboard. You can then paste it directly into Slack, Notion, Google Docs, GitHub PR comments, or any app that accepts pasted images — no file download needed.