PlantUML

PlantUML Sequence Diagram Editor — Online, No Setup

Import a PlantUML @startuml sequence diagram, edit it visually in your browser, and export back to PlantUML, Mermaid, PNG or SVG. No Java to install, no server to run, no account to create.

Open Editor Mermaid Editor

Why use an online PlantUML editor

PlantUML's standard workflow requires a Java runtime to render diagrams locally, or sending diagrams to an external server (plantuml.com). Neither is ideal when you want to sketch a quick flow, share it with a teammate, or export an image without setting anything up.

SeqDiagram runs entirely in your browser. Paste a PlantUML sequence diagram and it renders a live preview instantly — no Java, no server call, no install. When you're done, export to PlantUML to take the file back to your existing toolchain, or export to Mermaid, PNG, or SVG.

How import and export works

SeqDiagram does not render raw PlantUML syntax directly. It converts your PlantUML diagram to its native plain-text format, which you edit with a live preview, and then exports back. The native syntax is close to PlantUML's sequence notation, so most diagrams round-trip cleanly.

Open the editor at seqdiagram.com — no sign up needed.

Click Import → Import PlantUML in the toolbar. Paste your @startuml / @enduml block.

Edit the diagram in the text panel. The diagram re-renders live as you type.

Export when done. Use Export → Export PlantUML for a .puml file, Export Mermaid for .mmd, or the PNG/SVG buttons for images.

PlantUML syntax example

Here is a PlantUML sequence diagram for a user login flow. You can paste this directly into the Import PlantUML dialog:

@startuml
actor User
participant Browser
participant "Auth API" as API
participant Database

User -> Browser : Enter email + password
Browser -> API : POST /auth/login
API -> Database : Find user by email
Database --> API : User record
API -> API : Verify password hash
API --> Browser : 200 OK + JWT token
Browser --> User : Redirect to dashboard
@enduml

After importing, you can rename participants, add messages, reorder the flow, and export back to PlantUML or any other format.

What PlantUML syntax is supported

SeqDiagram's PlantUML importer handles the core sequence diagram constructs:

Participants and actors — participant Name, actor Name, aliased participants (participant "Long Name" as Short)
Message arrows — solid (->), dashed (-->)
Activation bars — activate / deactivate
Notes — note over, note left of, note right of
Loops and conditionals — loop, alt/else, opt
Title — title My Diagram

Where PlantUML diagrams render natively

Platform	How to use it
Confluence	PlantUML macro — renders natively, no third-party plugins needed
IntelliJ IDEA	PlantUML Integration plugin — renders inline while editing
VS Code	PlantUML extension by jebbs
GitHub	Does not render natively (use Mermaid for GitHub — see below)
GitLab	Does not render natively in markdown (use Mermaid instead)
Doxygen	Native support via the `@startuml` directive in comments

Converting PlantUML to Mermaid

If your diagram needs to end up in GitHub, GitLab, or Notion, you can convert it in two steps without any local tooling:

Import your PlantUML diagram via Import → Import PlantUML
Export via Export → Export Mermaid to download a .mmd file

The resulting Mermaid file uses standard sequenceDiagram notation and renders natively in GitHub markdown, GitLab, Notion, and Obsidian. See the Mermaid editor guide for more on where Mermaid renders.

Frequently asked questions

Can I edit PlantUML sequence diagrams without installing Java?

Yes. SeqDiagram runs entirely in your browser — no Java, no server, no install. Paste your @startuml / @enduml block and it converts automatically so you can edit and export without any local toolchain.

Does it export back to PlantUML?

Yes. After editing, use Export PlantUML in the toolbar to download a .puml file compatible with Confluence, JetBrains IDEs, and any other PlantUML-compatible tool.

What PlantUML syntax does this support?

SeqDiagram supports PlantUML sequence diagram notation including participants, actors, solid and dashed arrows (->, -->), activation boxes, notes, loop, alt/else, and opt fragments enclosed in @startuml / @enduml.

Can I convert a PlantUML diagram to Mermaid?

Yes. Import your PlantUML diagram via Import PlantUML, then use Export Mermaid to download a .mmd file compatible with GitHub, GitLab, Notion, and Obsidian. No local tooling needed.

Does SeqDiagram send my diagram to an external server?

No. Unlike the hosted plantuml.com renderer, SeqDiagram runs entirely in your browser. Your diagram content never leaves your machine unless you explicitly share it via the Share link feature.