PlantUML Sequence Diagram Editor — Online, No Setup

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

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.

Sketmi 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

Sketmi 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 the structure of a diagram (participants, messages, notes, fragments) survives the trip. Some details don't: participant icons, multi-line notes and styling are simplified or dropped, as listed under what is supported.

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 the PlantUML import supports, and what it doesn't

Sketmi handles PlantUML sequence diagrams only. Class, activity, component, state and other PlantUML diagram types are not supported.

Converted on import

• Participants and actors: participant, actor, and aliases written as participant "Long Name" as Short (the Short as "Long Name" order is not converted). boundary, control, entity, database and collections become plain participants, without their icons.
• Messages: ->, -->, ->>, -->>, and reversed arrows such as <- and <--.
• Notes: note over, note left of, note right of, on one line or as a note … end note block (its lines are joined into one).
• Fragments: loop, alt/else, opt, group, par, break, critical.
• Title: title My Diagram.

Dropped or not converted

• activate, deactivate and destroy lines are dropped. Add activation bars afterwards with the editor's own syntax (see the syntax guide).
• Messages that use -|>, --|>, -o or --o are currently lost, and lost-message arrows such as ->x and ->o are not converted. Change them to -> or --> before importing.
• Styling and layout commands (skinparam, !theme, !pragma, hide, show, autonumber, autoactivate, newpage, header, footer), == separators == and single-line comments are ignored. Remove multi-line /' … '/ comments before importing: lines inside them are read as diagram text.
• Anything else, such as coloured arrows (-[#red]>), is not converted. Check the preview after importing.

Exporting to PlantUML

Export wraps the diagram in @startuml / @enduml and keeps participants, messages, notes, fragments and the title. The +/- activation shorthand on arrows is removed, so those bars don't appear in the exported file; activate and deactivate lines are kept. Destroy arrows (-x, --x) are written as they are, while PlantUML's own notation for a lost message is ->x, so check those lines.

Where PlantUML diagrams render natively

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:

1. Import your PlantUML diagram via Import → PlantUML
2. Export via 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. Plain -> and --> arrows are written as they are, and Mermaid draws those without arrowheads; switch them to ->> and -->> if you want heads. See the Mermaid editor guide for more on where Mermaid renders.

Frequently asked questions

Related resources