PlantUML is an open-source tool that allows you to create UML diagrams from plain text descriptions. Using PlantUML to visualize JSON data structures provides a clean, standardized way to understand complex nested relationships at a glance. UML class diagrams are particularly effective for showing object hierarchies, data types, and cardinality relationships (one-to-one, one-to-many) that exist within your JSON schema.

This tool supports all standard JSON structures including: nested objects, arrays of primitives, arrays of objects, mixed-type arrays, null values, boolean values, numbers, and strings. Deeply nested structures (up to 20 levels) are rendered as separate interconnected classes. The root element can be either an object or an array. Each unique nested object type is extracted into its own UML class with appropriate relationship annotations.

You can copy the generated PlantUML source code and modify it directly. PlantUML supports extensive customization through skinparam directives (for colors, fonts, and styling) and theme imports. You can also open the diagram in the PlantUML Online Editor using the "Editor" button, where you'll have full access to all styling options and a live preview.

Your data stays in your browser. All JSON parsing and PlantUML code generation happens entirely on the client side using JavaScript. The diagram rendering uses the public Kroki.io API (which receives only the generated PlantUML markup, not your original JSON). For maximum privacy, you can copy the PlantUML code and render it locally using a self-hosted PlantUML server or offline rendering tool.

The tool provides SVG download for crystal-clear vector graphics that scale infinitely — perfect for documentation, presentations, and wiki pages. You can also copy the PlantUML source code and render it to PNG, PDF, LaTeX, or ASCII art using any PlantUML-compatible renderer. The "Editor" button opens your diagram in the PlantUML online editor where additional export formats are available.

Unlike JSON Schema tools that require a formal schema definition, this tool works directly with any JSON data sample — no schema required. It infers the structure from your actual data, making it ideal for reverse-engineering APIs, exploring unfamiliar datasets, or quickly documenting existing JSON structures. For formal schema documentation, consider combining this tool's output with JSON Schema generators.

The tool handles JSON files with up to 500+ properties and 20 levels of nesting. For extremely large datasets, consider using a representative sample or subset of your JSON structure. The resulting PlantUML diagram will faithfully represent all object types and relationships. If your JSON contains many repeated nested structures, they will be intelligently deduplicated into reusable class definitions.

Both are excellent diagramming tools. PlantUML excels at UML class diagrams with rich relationship annotations, multiplicity labels, and mature styling options — making it ideal for data structure visualization. Mermaid offers a simpler syntax and is natively supported in many Markdown platforms (GitHub, GitLab, Notion). PlantUML provides more comprehensive UML support, which is why we chose it for JSON structure visualization with proper cardinality and type annotations.