Structured Data Markup Validation and Testing Tools
Tools for testing structured data markup and rich snippet display, including microformats and schema.org marked up in microdata, RDFa, RDFa Lite and JSON-LD.
When testing vocabularies and schemas, note that error messages from validators may be related to the usage requirements of only that specific data consumer, especially validation error pertaining to required fields or required properties.
Main Multi-Format Validators
Google Structured Data Testing Tool
Google Webmaster Tools
Google's tool for validating schema.org, microformats and other structured data, and for generating previews of rich snippets in Google search results for supported types. Handles HTML marked up with microdata, RDFa and RDFa Lite.
Yandex Structured Data Testing Tool
Yandex's tool for validating schema.org, microformats, JSON-LD and other structured data.
Structured Data Linter
Validation and visualization tool for microdata, JSON-LD and RDFa, along with vocabulary validation support for schema.org, FOAF, SIOC, SKOS and Dublin Core (DCMI).
Bing also provides a beta Markup Validator, but use of this tool requires you first to login to Bing Webmaster Tools. Note that once logged in a webmaster can use the Validator to check the markup of any URL, not just the URLs of domains belonging to that Webmaster Tools account.
Open Graph Tools
Google Validators for Specific Item Types
Email Markup Tester
Google Developers/Google Webmaster Tools
Validator for JSON-LD and microdata, providing properties for each entity and specific error messages for invalid input.
Corporate Contacts Markup Tester
Google's tool for validating schema.org/Organization and schema.org/ContactPoint markup, with JSON-LD support.
Event Markup Tester
Google's tool for validating schema.org/Event markup, with JSON-LD support.
Structured Data Conversion Tools and Utilities
Alex Stolz / Martin Hepp / Bene Rodriguez-Castro
A multi-format conversion tool for structured markup, providing conversions to and from RDFa, microdata, JSON-LD, RDF/XML, RDF/JSON and more. Conversion input can be a URI or text. Comes with a REST API for developers.
Matthis de Wit
"This extension lets you know if there is Microdata or JSON-LD on a website an shows it to you."